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Energy and water in Kuwait: A sustainability viewpoint, Part II
Desalination 230 (2008) 140–152
Energy and water in Kuwait: A sustainability viewpoint, Part II M.A. Darwisha*, A.M. Darwishb a
Mechanical Engineering Department, Kuwait University, POB 5969, Safat 13060, Kuwait Tel. +965 481-1188, ext. 5789; Fax: +965 484-7131; email: [email protected] b American University of Cairo, Cairo, Egypt Received 30 June 2007; Accepted 27 October 2007
Abstract In Kuwait, the daily consumption per capita of electric power is 14,000 kWh, and of desalted water is 600 L. These are among the highest in the world, and the total consumption of each is almost doubled every 10 years. The cogeneration power desalting plants CPDP producing these two commodities consumed about 54% of the total 150 millions barrels of fuel consumed in the year 2005. If these consumption and production patterns prevail, the fuel oil produced in the country can be fully consumed locally in 30 years, with nothing left for export, the main source of income. The picture can be changed if better desalted water and power production methods are used. These include changing the desalting method from multistage flash MSF known by its high energy consumption to the more energy efficient seawater reverse osmosis SWRO; and power production method of steam or gas turbine cycles to combined gas/steam turbine combined cycle known of its high efficiency. The energy consumed by the air conditioning AC systems should be reduced by using better codes of building insulation and more efficient AC systems. Other conservation methods to reduce water consumption and the energy consumed by transportation are outlined in this paper. Keywords:
Cogeneration power desalting plants; Steam turbine; Gas turbines; Combined cycles; Multi-stage flash desalting methods; Multi-effect desalting systems; Reverse osmosis desalting methods; Wastewater treatment; Power consumption; Air conditioning energy consumption; Transportation
1. Introduction In part I of this study [1], the sustainability problem of water and energy in Kuwait was outlined. Part I showed that natural (ground) water resources which accumulated over centuries are almost exhausted. The consumption of desalted *Corresponding author.
water and electric energy is among the highest in the world. If the same rates of consumed power and water prevail, the country’s main source of income, the fuel oil produced can be fully consumed locally, with nothing left for export. However, the picture can be changed if conservation measures are followed. By reviewing the current practices of producing and consuming
0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.10.019
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electric power and desalted water, it is clear that some of these practices should be stopped, others can be modified, and more energy efficient methods should be adopted in future plans. This paper outlines some of the needed measures to save both water and energy. Many countries concerned about saving their resources and environments follow similar measures. 2. Desalted water and electric power production — current practices In 2004, Kuwait’s total energy consumption was about 150 million barrels per day (M-bbl/d). The cogeneration power–desalting plants CPDP consumed about 54% of the total consumed fuel, 63% of it as oil, 20% as gas, and 16% as crude oil. Kuwait needs to import natural gas to meet the growing demand of CPDP and to release more crude oil and its by-products for export. Gas imports are in the range of 200,000–
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220,000 bbl/d of energy equivalent. The present practice used in the CPDP is discussed first, before presenting some changes which can reduce the consumed energy. In Kuwait, the only method used to desalt seawater is the multi-stage flash (MSF) system, known for its high energy consumption compared to other desalting methods. The MSF units have to be combined with steam turbines to secure their required steam at moderate pressure (say at 2–3 bar) by extraction from turbines (Fig. 1). The extracted steam condenses in the MSF brine heater, and thus gives its latent heat to the desalting process. The steam condensate returns to the power cycle by joining the water feed to the boiler through a de-aerator. Kuwait’s main power plants are of CPDP type and similar repetitive units. Each unit consists of a 300-MW steam turbine combined with two MSF desalting units. Each MSF unit has a 7.2 MIGD capacity. One MIGD is equal to 4546 m3/d (or 52.62 kg/s).
Fig. 1. Dual-purpose power desalting plant using steam turbines and MSF desalting units.
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2.1. A bad oractice to be avoided The saturation temperature of steam supplied to the MSF units should be slightly higher (5– 7°C) than the top brine temperature (TBT) of about 110°C. So this steam should be at intermediate pressure (IP) of 2–3 bar. This steam can be supplied directly to the MSF units from fuel fired boilers, as in the Shuwaikh desalting plant. It can also be supplied by extraction from steam tur-bines combined with the MSF units as in CPDP (Fig. 1) In the first case (see Fig. 2), high availability fuel energy generates steam at IP as required to the desalting units. This is a bad practice that should be avoided. It is better to generate steam at high pressure (HP) and temperature as required in power plants, expand it in steam turbine to the pressure required by the desalting units. Thus, the steam produces power before it is supplied to the desalting units. This practice is used in co-generation power desalting plants. Thus more fuel energy is consumed when steam is directly supplied from boiler (its equivalent mechanical work is in the range of 40 kWh/m3 of desalted water), compared to the
fuel energy consumed if the steam is extracted from turbines (its equivalent mechanical work is in the range of 22 kWh/m3). In the winter, the electric load is low and the number of operating turbines may not be enough to supply steam to the MSF units. In this case, steam is supplied directly from the CPDP boilers to the MSF units, after throttling and de-superheating. More than 15% of distilled water is produced with steam supplied directly from steam generators [2]. 2.2. Substituting the MSF units with more energyefficient desalting methods The steam, extracted from the steam turbine to the two MSF units producing 12 MIGD in the Doha West plant, for example, could produce much more desalted water if the old MSF units were substituted by more energy-efficient desalting systems. These energy-efficient systems are the low temperature multi-effect desalination (LT–MED) units and the seawater reverse osmosis (SWRO) desalting systems [3]. The steam extracted to the two MSF at pressure higher than
Fig. 2. Single-purpose desalting plant, directly operated by fuel fired boilers.
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3 bar could be supplied to a back pressure steam turbine (BPST) to expand and thus produce power before leaving the turbine and directed to the LT–MED units at about 0.7 bar as required by these units. The BPST power output could be used to operate an SWRO plant producing 33.8 MIGD, while the LT–MED could produce 11.24 MGD. The suggested modification increases the desalted water from 12 to 45 MIGD and decreases the equivalent mechanical energy consumption from 22.3 kWh/m3 to 7.16 kWh/m3. 2.3. Using more efficient power and desalting systems To be more specific and show an energyefficient alternative to the current practice, a unit from the CPDP in Kuwait is used here as a reference unit (Fig. 1). The consumed fuel energy by the reference unit and its pollution to the environment are calculated. Then, a more efficient alternative producing the same power and desalted water is presented to show how much fuel energy can be saved and how its impact on the environment can be reduced for the same power and desalted water output. The reference unit from the present situation: A unit of the modern Azzour CPDP plant operating in Kuwait is used as a reference unit. The plant has 8 similar steam turbines, and each turbine is combined with two MSF units. The steam turbine nominal capacity is 300 MW electric power, and supplying thermal energy Qd = 196 MW to the brine heaters (BH) of the two MSF desalting units combined with the turbine. Each MSF unit has a 7.2 MIGD capacity. Thus the total desalted water is 14.4 MIGD (757.7 kg/s). The nominal power-to-desalted heat ratio is W/Qd = 1.53 MW/MW. In fact, the increasing rate of the desalted water demands is always higher than that of electric power. The real demand ratio of desalted water to power is higher than that ratio given by the design of the CPDP units. The turbine has a tandem
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arrangement with high pressure turbine (HPT), intermediate pressure turbine (IPT), and low pressure turbine (LPT) cylinders along with the generator mounted on a single shaft. The steam is extracted to the MSF units from cross pipe connecting the IPT and LPT cylinders. The cycle has regenerative feed heaters (5 closed and one open) and reheating. The flow sheet and state numbers are given in Fig. 3. The main plant data with and without steam supply to the MSF units at this loads are given in the appendix. In a CPDP, the steam generator (SG) is supplied by fuel energy equal to Qf = Mf HHV where Mf is the fuel mass flow rate supplied to the SG and HHV is the fuel high heating value. The efficiency of the SG is: ηb = heat gained by the water Qb/fuel energy Qf where Qb = Ms× [h (at throttling)!h (at feed to SG)] + Mr×[(h (hot reheat)!h (cold reheat)] where Ms is the steam flow rate to the HP turbine, Mr is the steam flow rate to the re-heater and to the IP turbine,and h is the enthalpy of the steam or water at the feed to the SG. By taking the boiler efficiency as ηb = 0.9 and the calculation of Qb, and Qf gives: Qb = 804.871 MW, and Qf = 894.30 MW. The conditions of the heating at the desalting inlet and outlet are given in Table 1. So, the heat given to the desalting plant Qd is: Qd = Sd (his!hwe) = 196 MW where Sd is the mass flow rate of the steam Table 1 Condition of heating at the desalting inlet and outlet State points on cycle
Desalting unit
Mass, kg/s
Enthalpy h, kJ/kg
7d in Fig. 1 16 in Fig. 1
Inlet Exit
76.717 76.717
2960.9 406.1
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Fig. 3. Suggested modifications: both SWRO and BPST are added, replacing MSF by MED units.
supplied to the desalting units of enthalpies his at the inlet and hwe of its condensate at the exit. The work loss due to the extraction of steam to the MSF units, instead of its expansion in the LPT, is called the work equivalent of the thermal energy supplied to the desalting units Wd, and is equal to 47.28 MW, and the equivalent specific work is 62.4 kJ/kg of desalted water. Thus, the total steam turbine equivalent power output can be calculated as 347.28 MW, and it consists with the known electric power output We = 300 MW, plus Wd the power equivalent to thermal energy Qd supplied to the MSF units, Wd = 47.28 MW. This gives plant efficiency equal to 347.28/894.3 = 0.39 = 39%. The fuel energy to the CPDP is allocated to power and water based on We and Wd. This means that: Fuel energy charged to desalination = (47.28/347.28) × 894.3 = 121.45 MW and Fuel charged to power = 894.3 – 121.45 = 772.85 MW
This means that only an additional 121.45 MW of fuel energy is added to CPDP to enable the supply of 196 MW to the MSF units, and this is the main merit of the CPDP. Besides the heat supplied to the MSF brine heaters, high pressure steam is supplied to the MSF steam ejectors (about 6% of the steam added to the BH) and pumping work energy at the rate of 14.4 kJ/kg (4 kWh/m3 of desalted water) to the MSF pumps to move its streams. If the pumping work is produced by a power plant of 0.39 efficiency, then the fuel energy charged for the pumping is 14.4/ 0.39 = 37 kJ/kg (37 MJ/m3). So, the fuel energy charged to desalt 1 m3, when the ejector steam and pumping energy are considered, is 207 MJ/m3. This gives specific equivalent work charged to desalt 1 m3 as 80.3 MJ/m3 (22.3 kWh/m3). The thermal energy and the flow rates of the streams rejected to sea from the reference unit are calculated by the same method used in Part I of the paper, and the results are given in Table 2. To decrease the effect of the high brine salinity on the sea environment, the rejected brine to the sea is usually mixed with the rejected cooling
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Table 2 Thermal energy and salinity of stream discharged from the reference unit Stream
Condenser cooling water MSF cooling water Brine blow down Total
ΔTM above seawater (EC)
7 11 11 8.183
Salinity (g/l)
Xf = 46 Xf = 46 Xb = 69 48.7
seawater (at seawater salinity) from both the power plant condenser and the MSF heat rejection sections. The mixture has lower concentration and temperature than those of the MSF blowdown brine. The mixture salinity (X) is calculated by dividing the sum of each stream by its salinity to the sum of the flow rates. X(discharge) = [(9021×46) + (2272×46) + (1515×69)]/(9021+2272+1515) = 48.7 g/l or only (48700 - 46000=) 2700 ppm (2.7 g/l) above the seawater salinity, and thus reduces its effect on aquatic life. The same can be done to obtain the mixture temperature: T(discharge) = [(9021×34) + (2272×38) + (1515×38)]/(9021+2272+1515) = 35.2°C. The picture is different in the winter when low seawater temperatures prevail and the electric load is low. In this case the cooling water flow rate to both the condenser and the HRJ are lower than in summer, and the mixed stream exit salinity can easily reach 5000–6000 ppm above that of the seawater, and temperature can easily reach 15°C above that of the seawater temperature. The brine blow-down has higher salinity Xb, which is calculated from the feed water to the
Single unit 300 MW
Doha West plant 8×300 MW turbine
Turbine, 14.4 MIGD
8×14.4 MIGD
Flow rate (kg/s)
Heat rejected (MW)
Flow rate (1000 ton/d)
Heat rejected (GW)
9,021 2,273 1,515.70 12,810
252.6 100 65 417.6
779.4 196.4 131 1,106.80
20.2 8 5.2 33.4
distillate ratio by F/D = Xb/(Xb!Xf) If Xf = 46 g/l, then Xb = 69 g/l. Note that the heat rejection to the sea and the heat contained with the distillate stream should be equal to the heat supplied to the BH. The heat contained in the distillate stream = 757.7(kg/s) × 4.186(kJ/kg °C) × 11(°C)/1000 = 34.9 MW. Hence, the total heat leaving the MSF units is 100 + 65 + 34.9 = 199.9 MW. Notice that this is a little higher than the 196 MW thermal energy supplied due to the pumping energy consumed to move the streams. The thermal pollution dumped to the sea from this CPDP unit and from the whole Doha West plant of 8 units are summarized in Table 2. Beside the heat rejected to sea, heat is rejected to the atmosphere from the steam generator SG. Since the SG efficiency is taken equal to 0.9, 10% of the fuel energy is rejected to air, or 89.43 MW. When oil of low heating value (LHV) = 40,000 kJ/kg is used as fuel, the oil flow rate is 894.3/40 = 22.36 kg/s. By assuming that 85% of the fuel is carbon and 3% is sulfur, the emission of CO2 to atmosphere would be 69.7 kg/s (2.2 Mt/y), and that for SO2 is 1.3416 kg/s (42309 t/y). The corresponding emission per kWh power is 0.695 kg CO2/kWh, and 0.0134 kg
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Fig. 4. Combined cycle of gas–steam turbines.
SO2/kWh, and emission per m3 desalted water is 15.547 kg CO2/m3 and 0.3 kg SO2/m3. If the Doha West plant of 8 units, similar to the reference unit considered in the previous example, was operating in one of the European countries, where a tax of $10/t of CO2 emitted is imposed, the annual CO2 tax for this plant would be: 8 units × 2.2 Mt CO2/y × $10/t CO2 = $17.6 million dollars and the CO2 tax per m3 of desalted water would be $0.16/m3. 2.4. Alternative more efficient energy option If a more energy-efficient combined cycle of gas/steam turbine (CC–GT/ST) power plant with 52% efficiency (Fig. 4) and a seawater reverse osmosis (SWRO) desalting plant (Fig. 5) consuming 5 kWh/m3 are used in place of the reference unit of the previous example, less fuel
energy would be consumed as well as rejected. The combined cycle can include more than a gas turbine GT, and each GT can be combined with heat recovery steam generator HRSG, and can drive an electric generator. The steam from (one or more) HRSG drives the bottoming steam turbine cycle driving an electric generator. A supplementary-fired boiler can be used to increase the steam production. While the thermal efficiency of GT is in the range of 30–38%, the CC GT/ST efficiency is in the range of 46–60%. An example of a commercial CC GT/ST cycle is considered here. It has three typical gas turbines (GT) of 80 MW each at ISO conditions (of 15EC ambient temperature) combined with three HRSG, which supply steam to a 100-MW steam turbine. Hence, the total power output is 340 MW. The efficiency of the GT is 0.37. The flue gases from each GT (240.4 kg/s at 457°C) act as heat supply to a HRSG. The electric power required to desalt 14.4 MIGD (757.7 kg/s) by SWRO consuming 5 kWh/m3 (18-kJ/kg) is 13,638 kW = 13.638 MW. So the net power
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Fig. 5. Schematic diagram of the seawater reverse osmosis desalting system.
output is 340!13.64 = 326.36 MW, if 14.4 MIGD are produced. If 300 MW power output is required, as the reference plant, the desalted water produced by the SWRO system is 42.23 MIGD, 193% more than the reference plant. The only fuel added is that to the three GT = 3×80/0.37 = 648 MW. Consequently, the CCG/S power plant efficiency is 52.45%. The heat rejected to the atmosphere by the flue gases leaving the HRSG chimneys at 153°C is 3×240.4×1.01×(153–40) = 82.31 MW. So, the heat supplied to the three HRSG is 3×240.4×1.01×(547–153)/100 = 289.9 MW. This gives (100/289.9=) 34.5% steam cycle efficiency, and 189.9 thermal energy rejected by the steam cycle condenser to the sea. The required fuel oil is 16.2 kg/s if the LHV = 40 MJ/kg. The generated CO2 is 50.49 kg/s (1.59 Mt/y), or 0.535 kg CO2/kWh, and 2.674 kg CO2/m3 desalted water. The generated SO2 is 0.972 kg/s, or 0.0103 kg SO2/kWh, and 0.0514 kg SO2/m3 desalted water. The annual CO2 tax of 8 similar units (of the same Doha West power plant capacity), and $10/CO2 t of CO2 = $127 millions. A comparison of the current practice and the suggested option is given in Table 3. The above comparison is not intended to give absolute figures but simply to evaluate the order of magnitude and the environmental impact related to the concerned desalination process. The consumed fuel energy in the CC GT/ST (the energy efficient option) is 72% of the reference cycle. The thermal pollutions to air and seawater are much more in the reference unit, as well the
flue gases, than in the alternative plant. Many power-producing and desalting arrangement systems which are more efficient can be suggested, but the main idea is to use the most efficient combined gas/steam combined power cycle, and the most efficient SWRO system. 3. Developing more water by wastewater reclamation 3.1. Wastewater reclamation Treated wastewater for reuse became a common practice worldwide. Wastewater is a water resource that should be fully utilized specially in arid areas like Kuwait. Its treatment cost for reuse as grey water or even with potable quality is much lower than the cost of desalting high salinity brackish or seawater. It is an available water resource that exists in urban communities, increases with the increase of inhabitant numbers, per capita consumption, and standard of living. Untreated wastewater generally contains high levels of organic material, numerous pathogenic micro-organism, as well as nutrients and toxic compounds. So, it must be conveyed away from its source, treated appropriately before final disposal, usually to the sea in Kuwait. This is necessary to protect the environment and avoid public health concerns. The total wastewater collected, presently in Kuwait is about 450,000 m3/d (about half the municipal water supply). Primary treatment involves partial removal of suspended solids
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Table 3 Comparison of the current practice and the suggested option Item
Case 1
Case 2
Net power output, MW Desalted water output, MIGD (kg/s) Other net power output option, MW Desalted water output option, MIGD (kg/s) kWh/m3 of desalted water Fuel energy input, MW Energy rejected to air, MW Streams rejected to sea Cooling condenser seawater flow rate kg/sa Cooling condenser energy rejected to sea, MW Desalting units streams HRJ seawater cooling flow rate, kg/s HRJ seawater cooling heat rate, MW Brine blow-down flow rate, kg/s Brine blow-down energy rejected, MW Brine blow-down salinity, g/l kg CO2/kWh kg CO2/m3 desalted water Million tons of CO2 kg SO2/kWh kg SO2/m3 desalted water
300 14.4 (757.7) 300
326.23 14.4 (757.7) 300
14.4 (757.7) 22.37 894.3 89.42
42.3
9021
6782
252.6
189.9
2273
NA
100
NA
1515.7 65
1515.7 Almost 0 69 0.535 2.674 1.59 0.0103 0.0514
69 0.695 15.547 2.2 0.0134 0.3
5 648 82.31
a
Seawater salinity.
and organic matter by physical operations such as screening, grit removal, pre-aeration using floating aerators, primary aeration using bubble aeration, and primary settling. The secondary treatment includes second aeration using extended bubble and mechanical agitation, and secondary settling. The tertiary treatment includes balancing tanks with chlorination, sand filtration, and secondary chlorination. The sludge is treated
by pre-thickening, digestion, post thickening, and drying in beds (Fig. 6). Part of the tertiary treated effluent is reused for irrigation, and the balance is dumped to the sea. The tertiary treatment goes beyond the level of conventional secondary treatment to remove significant amounts of nitrogen, phosphorus, heavy metals, biodegradable organics, bacteria and viruses. In addition to biological nutrient removal processes, unit operations frequently used for this purpose include chemical coagulation, flocculation and sedimentation, followed by filtration and activated carbon. The additional filtration–disinfection steps (tertiary treatment) are applied for unrestricted agricultural or landscape irrigation as well as for process water in some industrial applications. In Kuwait, the product after the tertiary treatment has relatively good quality, and is discharged to the sea except for a small quantity used for limited agricultural purposes. It has relatively low salinity (1000 mg/l) when compared with brackish water (4000– 5000 mg/l), and makes it potentially good quality water source for final pretreatment. The over 100 MIGD of treated wastewater has a great potential, if finally treated, to supplement or replace the brackish water supplies and might be the solution to re-addressing the balance of demand for irrigation water and complement the need for more water supplies in Kuwait [4]. The dissolved organics and other contaminants present in the tertiary treated effluent are known or suspected to be detrimental to various reuse applications and still limit full utilization of this valuable resource. What is called quaternary treatment is defined and used as the treatment producing potable water quality to meet unrestricted residential uses and industrial applications requiring ultra-pure water. Membranes of different pore sizes are usually used in this process such as micro-filtration (MF), ultra-filtration (UF), nano-filtration (NF), and hyper-filtration reverse osmosis (RO) in descending pore diameter order. As a general rule, MF is suitable for
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Fig. 6. Water distribution in Kuwait.
the removal of suspended solids, including larger micro-organisms like protozoa and bacteria. UF is required for the removal of viruses and organic macromolecules down to a size of around 20 nm. Smaller organics and multivalent ions may be removed by NF while RO is even suitable for the removal of all dissolved species [5]. However, direct human consumption of this treated effluent may be objectionable for psychological and probably religious reasons, although it can satisfy the potable water requirements and used for human consumption in many parts of the world. The quaternary treated waters are used as direct potable water or recharged to aquifers for storage and then extracted for potable purposes in many parts of the world. A new wastewater treatment and reclamation
plant (WWRP) was built in Sulaibiya, Kuwait, and uses UF and RO to produce potable water quality from the tertiary treated municipal wastewater. The RO removes dissolved salts and harmful contaminants, including bacteria, viruses and chemicals. The WWRP is the largest facility of its kind in the world to use RO and UF membranebased water purification. The plant’s initial daily capacity is 375,000 m³/d and designed for extension to 600,000 m³/d in the future. When fully operational, the facility is predicted to contribute 26% of Kuwait’s overall water demand, reducing the annual demand from non-potable sources from 142 Mm3 to 26 Mm3. The plant water production satisfies the increasing agricultural, industrial and domestic demands and can preserve the natural strategic water resources. The
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plant also reduces the environmental pollution resulting from the direct discharge of secondary– tertiary municipal effluents into the sea. The Ministry of Energy is financing a research project to artificially recharge the Sulaibiya plant output potable quality water in ground water aquifers. The conventional treatment cost is $0.36/m3, which represents less than 10% of the cost of MSF distillate. 3.2. Water demand management Water consumption is really high (600 L/d per capita) in Kuwait because (a) no metering is used, (b) consumers have high income, (c) the water price is very low and is based on a flat rate, (d) weather is hot and dry, and (e) consumers do not recognize the value of water. It takes two sides to conserve water: the consumers and authorities (like municipalities and the Ministry of Electricity and Water). So, there are apparent measures that should be considered such as: 1. Inform, encourage, and teach consumers how to use water efficiently and enforce necessary measures to achieve that; regulate and restrict specific water uses; and develop a public education program. 2. Metering water at the source and points of use, detect leakage to decrease unaccounted for water (up to 50% of water loss are due to leakages in many countries), and repair water lines. 3. Charge fees according to amount used by consumers beyond their basic needs. 4. Invest in appliances, processes, and technologies that can reduce consumed water. 5. Audit water in housing, public buildings, commercial, and industrial sectors should be conducted to know where conservation measure should be applied. 6. Subsidize or supply free of charge only the basic water needs of customers, but beyond that, the exact cost of water should be charged. 7. Apply known conservation methods used in housing such as the use of (a) low-flow plumbing
fixtures, use of pressure reduction devices, and low-flow shower-heads, (b) toilet displacement devices to reduce the amount of water used by flush, e.g., placing a milk jug filled with water in the tank., (c) replacing conventional toilets, which use 15–20 L/flush with low flush toilets which use 6 L/flush or less, (d) stopping the use of hoses in car washing, and sweeping side walks and driveways. Water users (households, agriculture and industry) should be consciously oriented to save water. 3.3. Fuel energy consumed by cars Cars are another big fuel consumer. Cars are also a big source of air pollution due to fuel burning, which emits CO2, SO2, NOx, CO, hydrocarbons (fuel vapor when exposed to air), and particulates. Table 4 gives the fuel consumption and the pollutant of a car driven an average 15,000 km/y. There are about one million cars registered in Kuwait and a good fraction of them are big cars. For an average car traveling 15,000 km/y, the emission rates of different gases and consumption in g/l and in kg/y are shown in Table 4. The main reason of high fuel consumption by cars compared to other transportation methods is the low number occupants traveling in each car, as shown in Table 5. There are simple rules to decrease the energy consumption and polluted gases emitted by cars and these include: 1. Use efficient and small cars and keep them under best performance conditions. The government should tax large cars. 2. Use car pools with as many partners as possible. 3. Use public transportation like buses or trains. Kuwait has a good public bus system, but it needs more lines, and a train system. The public transportation system including railways and more bus lines should be expanded by the government.
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Table 4 Emission rates of different gases, and consumption in g/l and in kg/y for an average car traveling 15,000 km/y Component
Emission rate and consumed fuel
Calculation
Total annual pollution emitted and fuel consumed
CO NOx CO2 Gasoline
1.68 0.83 256.90 0.1044
25.15 12.49 3853.45 1566.00
kg/y kg/y kg/y L/y
(g/km)×15,000/1,000 (g/km)×15,000/1,000 (g/km)×15,000/1,000 (l/km)×15,000
Table 5 Energy use in MJ per passenger per mile Transport mode
Number of people carried
Energy use (MJ) per passenger mile
Large car (in town) Large car (open road) Small car (in town) Small car (open road) Train (intercity) Train (commuter) Bus Motorcycle Walking Cycling
1.5 1.5 1.5 1.5 50% full 65% full 50% full 1 1 1
6.8 3.9 3.8 2.7 2.1 0.9 1 4.3 0.4 0.1
3.4. Electric power demand management Air conditioning systems in Kuwait consume about 75% of the electric energy generated at peak load time. Reduction of the energy consumed by A/C will reduce the peak load. In Kuwait, the price of electric energy is highly subsidized (93%). It is sold for 2 fils/kWh, while its production cost is 30 fils/kWh. Due to this low price, there are no real public incentives to save energy. Therefore, the government should enforce saving energy by imposing codes or regulations, especially in the A/C area. The only measures imposed by the Ministry of Energy to save energy are:
1. Setting maximum wattage/m2 of conditioned spaces when current is connected to buildings; 2. The use of water cooled condensers in A/C units with capacity higher than 500 t (1750 kW) cooling. 3. Minimum coefficient of performance (COP) is imposed on imported A/C equipment, such as 1.75 for air-cooled and 2.5 for watercooled condensers, including all electric-driven devices. In addition, non-mandatory guidelines are applied, and include the following: 1. Walls and roofs should have minimum resistance of 1.76 m2 °C/W and 2.54 m2 °C/W, respectively. 2. Maximum glazing to wall area ratio is 10% for ordinary glass and 15% for double-glazing. 3. Maximum allowable outside air is one air change/hour. In fact, these measures, except the insulation required for walls and roofs, are far from ensuring efficient A/C systems. The ASHRAE issued new standards providing minimum requirements for saving energy in new buildings or new additions to existing buildings in USA. At least these standards should be adopted and enforced in Kuwait. The ASHRAE standards (ASHRAE/ INSA 90 2002) are stricter than the Ministry of Energy and Water (MEW) guidelines proposed in the early 1980s. An example is given here [6]: the cooling load for a typical 265 m2 area villa was
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calculated as 46.82 kW per villa. The rate of consumed electric energy is 26.75 kW, based on the mandated 1.75 COP by the MEW. When the walls and roof were insulated according to the ASHRAE code, the cooling load for the 265 m2 was reduced to 30 kW. The minimum COP of air-cooled package and split A/C systems by ASHRAE standards is 2.93 at US-rated conditions. This can be corrected to 2.5 for Kuwaiti conditions (compared with 1.75 suggested by MEW). For the considered villa of 265 m2 area, the use of an efficient A/C system with minimum COP of 2.5 gives the consumed electric energy equal to only 12 kW when the load is decreased to 30 kW. This shows that by raising the insulation of walls and roofs and using more efficient A/C system, the electric consumption of the A/C was reduced to less than half. References [1] M.A. Darwish, F. Al-Awadi and A.M. Darwish, Energy and water in Kuwait: A sustainability viewpoint, Part I, Desalination, 225 (2008) 341–355. [2] M.A. Darwish, S. Al-Otaibi and K. Al-Shaigi, Suggested modifications of power-desalting plants in Kuwait, Desalination, 216 (2007) 222–231. [3] M.A. Darwish and N. Al Najem, The water problem in Kuwait, Desalination, 177 (2005) 167–177. [4] M. Abdel-Jawad, S. Ebrahim, M. Al-Tabtabaei and S. Al-Shammari, Advanced technologies for municipal wastewater purification: technical and economic assessment, Desalination, 124 (1999) 251–261. [5] T. Wintgens, T. Melin, A. Schafer, S. Khan, M. Muston, D. Bixio and C. Thoeye, The role of membrane processes in municipal wastewater reclamation and reuse, Desalination, 187 (2005) 1–11.
Appendix Table A1 Main data for the CPDP (with and without steam extracted to desalting unit) in Doha West, Kuwait Nominal capacity, 300 MW
Steam No steam extracted to extracted desalters to desalters
Output steam flow rate (kg/s) Throttle pressure (bar) Throttle temperature (C) Reheat pressure (bar) Reheat temperature (C) Final feed temperature (C) Condenser pressure (bar) Extracted steam flow rate to desalting unit (kg/s) Extracted steam pressure (bar) Extracted steam temperature (C) Enthalpy of steam inlet to desalting unit (kJ/kg) Enthalpy of condensate leaving the desalting unit (kJ/kg) Heat flow rate (kJ/s)
297.6
261.037
139 535 38.4 535 246.4 0.0637 76.72
139 535 36.7 485 246.1 0.0851
4.74 249.5 2960.9 406.1
195,997
Energy and water in Kuwait: A sustainability viewpoint, Part II M.A. Darwisha*, A.M. Darwishb a
Mechanical Engineering Department, Kuwait University, POB 5969, Safat 13060, Kuwait Tel. +965 481-1188, ext. 5789; Fax: +965 484-7131; email: [email protected] b American University of Cairo, Cairo, Egypt Received 30 June 2007; Accepted 27 October 2007
Abstract In Kuwait, the daily consumption per capita of electric power is 14,000 kWh, and of desalted water is 600 L. These are among the highest in the world, and the total consumption of each is almost doubled every 10 years. The cogeneration power desalting plants CPDP producing these two commodities consumed about 54% of the total 150 millions barrels of fuel consumed in the year 2005. If these consumption and production patterns prevail, the fuel oil produced in the country can be fully consumed locally in 30 years, with nothing left for export, the main source of income. The picture can be changed if better desalted water and power production methods are used. These include changing the desalting method from multistage flash MSF known by its high energy consumption to the more energy efficient seawater reverse osmosis SWRO; and power production method of steam or gas turbine cycles to combined gas/steam turbine combined cycle known of its high efficiency. The energy consumed by the air conditioning AC systems should be reduced by using better codes of building insulation and more efficient AC systems. Other conservation methods to reduce water consumption and the energy consumed by transportation are outlined in this paper. Keywords:
Cogeneration power desalting plants; Steam turbine; Gas turbines; Combined cycles; Multi-stage flash desalting methods; Multi-effect desalting systems; Reverse osmosis desalting methods; Wastewater treatment; Power consumption; Air conditioning energy consumption; Transportation
1. Introduction In part I of this study [1], the sustainability problem of water and energy in Kuwait was outlined. Part I showed that natural (ground) water resources which accumulated over centuries are almost exhausted. The consumption of desalted *Corresponding author.
water and electric energy is among the highest in the world. If the same rates of consumed power and water prevail, the country’s main source of income, the fuel oil produced can be fully consumed locally, with nothing left for export. However, the picture can be changed if conservation measures are followed. By reviewing the current practices of producing and consuming
0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.10.019
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electric power and desalted water, it is clear that some of these practices should be stopped, others can be modified, and more energy efficient methods should be adopted in future plans. This paper outlines some of the needed measures to save both water and energy. Many countries concerned about saving their resources and environments follow similar measures. 2. Desalted water and electric power production — current practices In 2004, Kuwait’s total energy consumption was about 150 million barrels per day (M-bbl/d). The cogeneration power–desalting plants CPDP consumed about 54% of the total consumed fuel, 63% of it as oil, 20% as gas, and 16% as crude oil. Kuwait needs to import natural gas to meet the growing demand of CPDP and to release more crude oil and its by-products for export. Gas imports are in the range of 200,000–
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220,000 bbl/d of energy equivalent. The present practice used in the CPDP is discussed first, before presenting some changes which can reduce the consumed energy. In Kuwait, the only method used to desalt seawater is the multi-stage flash (MSF) system, known for its high energy consumption compared to other desalting methods. The MSF units have to be combined with steam turbines to secure their required steam at moderate pressure (say at 2–3 bar) by extraction from turbines (Fig. 1). The extracted steam condenses in the MSF brine heater, and thus gives its latent heat to the desalting process. The steam condensate returns to the power cycle by joining the water feed to the boiler through a de-aerator. Kuwait’s main power plants are of CPDP type and similar repetitive units. Each unit consists of a 300-MW steam turbine combined with two MSF desalting units. Each MSF unit has a 7.2 MIGD capacity. One MIGD is equal to 4546 m3/d (or 52.62 kg/s).
Fig. 1. Dual-purpose power desalting plant using steam turbines and MSF desalting units.
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2.1. A bad oractice to be avoided The saturation temperature of steam supplied to the MSF units should be slightly higher (5– 7°C) than the top brine temperature (TBT) of about 110°C. So this steam should be at intermediate pressure (IP) of 2–3 bar. This steam can be supplied directly to the MSF units from fuel fired boilers, as in the Shuwaikh desalting plant. It can also be supplied by extraction from steam tur-bines combined with the MSF units as in CPDP (Fig. 1) In the first case (see Fig. 2), high availability fuel energy generates steam at IP as required to the desalting units. This is a bad practice that should be avoided. It is better to generate steam at high pressure (HP) and temperature as required in power plants, expand it in steam turbine to the pressure required by the desalting units. Thus, the steam produces power before it is supplied to the desalting units. This practice is used in co-generation power desalting plants. Thus more fuel energy is consumed when steam is directly supplied from boiler (its equivalent mechanical work is in the range of 40 kWh/m3 of desalted water), compared to the
fuel energy consumed if the steam is extracted from turbines (its equivalent mechanical work is in the range of 22 kWh/m3). In the winter, the electric load is low and the number of operating turbines may not be enough to supply steam to the MSF units. In this case, steam is supplied directly from the CPDP boilers to the MSF units, after throttling and de-superheating. More than 15% of distilled water is produced with steam supplied directly from steam generators [2]. 2.2. Substituting the MSF units with more energyefficient desalting methods The steam, extracted from the steam turbine to the two MSF units producing 12 MIGD in the Doha West plant, for example, could produce much more desalted water if the old MSF units were substituted by more energy-efficient desalting systems. These energy-efficient systems are the low temperature multi-effect desalination (LT–MED) units and the seawater reverse osmosis (SWRO) desalting systems [3]. The steam extracted to the two MSF at pressure higher than
Fig. 2. Single-purpose desalting plant, directly operated by fuel fired boilers.
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3 bar could be supplied to a back pressure steam turbine (BPST) to expand and thus produce power before leaving the turbine and directed to the LT–MED units at about 0.7 bar as required by these units. The BPST power output could be used to operate an SWRO plant producing 33.8 MIGD, while the LT–MED could produce 11.24 MGD. The suggested modification increases the desalted water from 12 to 45 MIGD and decreases the equivalent mechanical energy consumption from 22.3 kWh/m3 to 7.16 kWh/m3. 2.3. Using more efficient power and desalting systems To be more specific and show an energyefficient alternative to the current practice, a unit from the CPDP in Kuwait is used here as a reference unit (Fig. 1). The consumed fuel energy by the reference unit and its pollution to the environment are calculated. Then, a more efficient alternative producing the same power and desalted water is presented to show how much fuel energy can be saved and how its impact on the environment can be reduced for the same power and desalted water output. The reference unit from the present situation: A unit of the modern Azzour CPDP plant operating in Kuwait is used as a reference unit. The plant has 8 similar steam turbines, and each turbine is combined with two MSF units. The steam turbine nominal capacity is 300 MW electric power, and supplying thermal energy Qd = 196 MW to the brine heaters (BH) of the two MSF desalting units combined with the turbine. Each MSF unit has a 7.2 MIGD capacity. Thus the total desalted water is 14.4 MIGD (757.7 kg/s). The nominal power-to-desalted heat ratio is W/Qd = 1.53 MW/MW. In fact, the increasing rate of the desalted water demands is always higher than that of electric power. The real demand ratio of desalted water to power is higher than that ratio given by the design of the CPDP units. The turbine has a tandem
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arrangement with high pressure turbine (HPT), intermediate pressure turbine (IPT), and low pressure turbine (LPT) cylinders along with the generator mounted on a single shaft. The steam is extracted to the MSF units from cross pipe connecting the IPT and LPT cylinders. The cycle has regenerative feed heaters (5 closed and one open) and reheating. The flow sheet and state numbers are given in Fig. 3. The main plant data with and without steam supply to the MSF units at this loads are given in the appendix. In a CPDP, the steam generator (SG) is supplied by fuel energy equal to Qf = Mf HHV where Mf is the fuel mass flow rate supplied to the SG and HHV is the fuel high heating value. The efficiency of the SG is: ηb = heat gained by the water Qb/fuel energy Qf where Qb = Ms× [h (at throttling)!h (at feed to SG)] + Mr×[(h (hot reheat)!h (cold reheat)] where Ms is the steam flow rate to the HP turbine, Mr is the steam flow rate to the re-heater and to the IP turbine,and h is the enthalpy of the steam or water at the feed to the SG. By taking the boiler efficiency as ηb = 0.9 and the calculation of Qb, and Qf gives: Qb = 804.871 MW, and Qf = 894.30 MW. The conditions of the heating at the desalting inlet and outlet are given in Table 1. So, the heat given to the desalting plant Qd is: Qd = Sd (his!hwe) = 196 MW where Sd is the mass flow rate of the steam Table 1 Condition of heating at the desalting inlet and outlet State points on cycle
Desalting unit
Mass, kg/s
Enthalpy h, kJ/kg
7d in Fig. 1 16 in Fig. 1
Inlet Exit
76.717 76.717
2960.9 406.1
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Fig. 3. Suggested modifications: both SWRO and BPST are added, replacing MSF by MED units.
supplied to the desalting units of enthalpies his at the inlet and hwe of its condensate at the exit. The work loss due to the extraction of steam to the MSF units, instead of its expansion in the LPT, is called the work equivalent of the thermal energy supplied to the desalting units Wd, and is equal to 47.28 MW, and the equivalent specific work is 62.4 kJ/kg of desalted water. Thus, the total steam turbine equivalent power output can be calculated as 347.28 MW, and it consists with the known electric power output We = 300 MW, plus Wd the power equivalent to thermal energy Qd supplied to the MSF units, Wd = 47.28 MW. This gives plant efficiency equal to 347.28/894.3 = 0.39 = 39%. The fuel energy to the CPDP is allocated to power and water based on We and Wd. This means that: Fuel energy charged to desalination = (47.28/347.28) × 894.3 = 121.45 MW and Fuel charged to power = 894.3 – 121.45 = 772.85 MW
This means that only an additional 121.45 MW of fuel energy is added to CPDP to enable the supply of 196 MW to the MSF units, and this is the main merit of the CPDP. Besides the heat supplied to the MSF brine heaters, high pressure steam is supplied to the MSF steam ejectors (about 6% of the steam added to the BH) and pumping work energy at the rate of 14.4 kJ/kg (4 kWh/m3 of desalted water) to the MSF pumps to move its streams. If the pumping work is produced by a power plant of 0.39 efficiency, then the fuel energy charged for the pumping is 14.4/ 0.39 = 37 kJ/kg (37 MJ/m3). So, the fuel energy charged to desalt 1 m3, when the ejector steam and pumping energy are considered, is 207 MJ/m3. This gives specific equivalent work charged to desalt 1 m3 as 80.3 MJ/m3 (22.3 kWh/m3). The thermal energy and the flow rates of the streams rejected to sea from the reference unit are calculated by the same method used in Part I of the paper, and the results are given in Table 2. To decrease the effect of the high brine salinity on the sea environment, the rejected brine to the sea is usually mixed with the rejected cooling
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Table 2 Thermal energy and salinity of stream discharged from the reference unit Stream
Condenser cooling water MSF cooling water Brine blow down Total
ΔTM above seawater (EC)
7 11 11 8.183
Salinity (g/l)
Xf = 46 Xf = 46 Xb = 69 48.7
seawater (at seawater salinity) from both the power plant condenser and the MSF heat rejection sections. The mixture has lower concentration and temperature than those of the MSF blowdown brine. The mixture salinity (X) is calculated by dividing the sum of each stream by its salinity to the sum of the flow rates. X(discharge) = [(9021×46) + (2272×46) + (1515×69)]/(9021+2272+1515) = 48.7 g/l or only (48700 - 46000=) 2700 ppm (2.7 g/l) above the seawater salinity, and thus reduces its effect on aquatic life. The same can be done to obtain the mixture temperature: T(discharge) = [(9021×34) + (2272×38) + (1515×38)]/(9021+2272+1515) = 35.2°C. The picture is different in the winter when low seawater temperatures prevail and the electric load is low. In this case the cooling water flow rate to both the condenser and the HRJ are lower than in summer, and the mixed stream exit salinity can easily reach 5000–6000 ppm above that of the seawater, and temperature can easily reach 15°C above that of the seawater temperature. The brine blow-down has higher salinity Xb, which is calculated from the feed water to the
Single unit 300 MW
Doha West plant 8×300 MW turbine
Turbine, 14.4 MIGD
8×14.4 MIGD
Flow rate (kg/s)
Heat rejected (MW)
Flow rate (1000 ton/d)
Heat rejected (GW)
9,021 2,273 1,515.70 12,810
252.6 100 65 417.6
779.4 196.4 131 1,106.80
20.2 8 5.2 33.4
distillate ratio by F/D = Xb/(Xb!Xf) If Xf = 46 g/l, then Xb = 69 g/l. Note that the heat rejection to the sea and the heat contained with the distillate stream should be equal to the heat supplied to the BH. The heat contained in the distillate stream = 757.7(kg/s) × 4.186(kJ/kg °C) × 11(°C)/1000 = 34.9 MW. Hence, the total heat leaving the MSF units is 100 + 65 + 34.9 = 199.9 MW. Notice that this is a little higher than the 196 MW thermal energy supplied due to the pumping energy consumed to move the streams. The thermal pollution dumped to the sea from this CPDP unit and from the whole Doha West plant of 8 units are summarized in Table 2. Beside the heat rejected to sea, heat is rejected to the atmosphere from the steam generator SG. Since the SG efficiency is taken equal to 0.9, 10% of the fuel energy is rejected to air, or 89.43 MW. When oil of low heating value (LHV) = 40,000 kJ/kg is used as fuel, the oil flow rate is 894.3/40 = 22.36 kg/s. By assuming that 85% of the fuel is carbon and 3% is sulfur, the emission of CO2 to atmosphere would be 69.7 kg/s (2.2 Mt/y), and that for SO2 is 1.3416 kg/s (42309 t/y). The corresponding emission per kWh power is 0.695 kg CO2/kWh, and 0.0134 kg
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Fig. 4. Combined cycle of gas–steam turbines.
SO2/kWh, and emission per m3 desalted water is 15.547 kg CO2/m3 and 0.3 kg SO2/m3. If the Doha West plant of 8 units, similar to the reference unit considered in the previous example, was operating in one of the European countries, where a tax of $10/t of CO2 emitted is imposed, the annual CO2 tax for this plant would be: 8 units × 2.2 Mt CO2/y × $10/t CO2 = $17.6 million dollars and the CO2 tax per m3 of desalted water would be $0.16/m3. 2.4. Alternative more efficient energy option If a more energy-efficient combined cycle of gas/steam turbine (CC–GT/ST) power plant with 52% efficiency (Fig. 4) and a seawater reverse osmosis (SWRO) desalting plant (Fig. 5) consuming 5 kWh/m3 are used in place of the reference unit of the previous example, less fuel
energy would be consumed as well as rejected. The combined cycle can include more than a gas turbine GT, and each GT can be combined with heat recovery steam generator HRSG, and can drive an electric generator. The steam from (one or more) HRSG drives the bottoming steam turbine cycle driving an electric generator. A supplementary-fired boiler can be used to increase the steam production. While the thermal efficiency of GT is in the range of 30–38%, the CC GT/ST efficiency is in the range of 46–60%. An example of a commercial CC GT/ST cycle is considered here. It has three typical gas turbines (GT) of 80 MW each at ISO conditions (of 15EC ambient temperature) combined with three HRSG, which supply steam to a 100-MW steam turbine. Hence, the total power output is 340 MW. The efficiency of the GT is 0.37. The flue gases from each GT (240.4 kg/s at 457°C) act as heat supply to a HRSG. The electric power required to desalt 14.4 MIGD (757.7 kg/s) by SWRO consuming 5 kWh/m3 (18-kJ/kg) is 13,638 kW = 13.638 MW. So the net power
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Fig. 5. Schematic diagram of the seawater reverse osmosis desalting system.
output is 340!13.64 = 326.36 MW, if 14.4 MIGD are produced. If 300 MW power output is required, as the reference plant, the desalted water produced by the SWRO system is 42.23 MIGD, 193% more than the reference plant. The only fuel added is that to the three GT = 3×80/0.37 = 648 MW. Consequently, the CCG/S power plant efficiency is 52.45%. The heat rejected to the atmosphere by the flue gases leaving the HRSG chimneys at 153°C is 3×240.4×1.01×(153–40) = 82.31 MW. So, the heat supplied to the three HRSG is 3×240.4×1.01×(547–153)/100 = 289.9 MW. This gives (100/289.9=) 34.5% steam cycle efficiency, and 189.9 thermal energy rejected by the steam cycle condenser to the sea. The required fuel oil is 16.2 kg/s if the LHV = 40 MJ/kg. The generated CO2 is 50.49 kg/s (1.59 Mt/y), or 0.535 kg CO2/kWh, and 2.674 kg CO2/m3 desalted water. The generated SO2 is 0.972 kg/s, or 0.0103 kg SO2/kWh, and 0.0514 kg SO2/m3 desalted water. The annual CO2 tax of 8 similar units (of the same Doha West power plant capacity), and $10/CO2 t of CO2 = $127 millions. A comparison of the current practice and the suggested option is given in Table 3. The above comparison is not intended to give absolute figures but simply to evaluate the order of magnitude and the environmental impact related to the concerned desalination process. The consumed fuel energy in the CC GT/ST (the energy efficient option) is 72% of the reference cycle. The thermal pollutions to air and seawater are much more in the reference unit, as well the
flue gases, than in the alternative plant. Many power-producing and desalting arrangement systems which are more efficient can be suggested, but the main idea is to use the most efficient combined gas/steam combined power cycle, and the most efficient SWRO system. 3. Developing more water by wastewater reclamation 3.1. Wastewater reclamation Treated wastewater for reuse became a common practice worldwide. Wastewater is a water resource that should be fully utilized specially in arid areas like Kuwait. Its treatment cost for reuse as grey water or even with potable quality is much lower than the cost of desalting high salinity brackish or seawater. It is an available water resource that exists in urban communities, increases with the increase of inhabitant numbers, per capita consumption, and standard of living. Untreated wastewater generally contains high levels of organic material, numerous pathogenic micro-organism, as well as nutrients and toxic compounds. So, it must be conveyed away from its source, treated appropriately before final disposal, usually to the sea in Kuwait. This is necessary to protect the environment and avoid public health concerns. The total wastewater collected, presently in Kuwait is about 450,000 m3/d (about half the municipal water supply). Primary treatment involves partial removal of suspended solids
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Table 3 Comparison of the current practice and the suggested option Item
Case 1
Case 2
Net power output, MW Desalted water output, MIGD (kg/s) Other net power output option, MW Desalted water output option, MIGD (kg/s) kWh/m3 of desalted water Fuel energy input, MW Energy rejected to air, MW Streams rejected to sea Cooling condenser seawater flow rate kg/sa Cooling condenser energy rejected to sea, MW Desalting units streams HRJ seawater cooling flow rate, kg/s HRJ seawater cooling heat rate, MW Brine blow-down flow rate, kg/s Brine blow-down energy rejected, MW Brine blow-down salinity, g/l kg CO2/kWh kg CO2/m3 desalted water Million tons of CO2 kg SO2/kWh kg SO2/m3 desalted water
300 14.4 (757.7) 300
326.23 14.4 (757.7) 300
14.4 (757.7) 22.37 894.3 89.42
42.3
9021
6782
252.6
189.9
2273
NA
100
NA
1515.7 65
1515.7 Almost 0 69 0.535 2.674 1.59 0.0103 0.0514
69 0.695 15.547 2.2 0.0134 0.3
5 648 82.31
a
Seawater salinity.
and organic matter by physical operations such as screening, grit removal, pre-aeration using floating aerators, primary aeration using bubble aeration, and primary settling. The secondary treatment includes second aeration using extended bubble and mechanical agitation, and secondary settling. The tertiary treatment includes balancing tanks with chlorination, sand filtration, and secondary chlorination. The sludge is treated
by pre-thickening, digestion, post thickening, and drying in beds (Fig. 6). Part of the tertiary treated effluent is reused for irrigation, and the balance is dumped to the sea. The tertiary treatment goes beyond the level of conventional secondary treatment to remove significant amounts of nitrogen, phosphorus, heavy metals, biodegradable organics, bacteria and viruses. In addition to biological nutrient removal processes, unit operations frequently used for this purpose include chemical coagulation, flocculation and sedimentation, followed by filtration and activated carbon. The additional filtration–disinfection steps (tertiary treatment) are applied for unrestricted agricultural or landscape irrigation as well as for process water in some industrial applications. In Kuwait, the product after the tertiary treatment has relatively good quality, and is discharged to the sea except for a small quantity used for limited agricultural purposes. It has relatively low salinity (1000 mg/l) when compared with brackish water (4000– 5000 mg/l), and makes it potentially good quality water source for final pretreatment. The over 100 MIGD of treated wastewater has a great potential, if finally treated, to supplement or replace the brackish water supplies and might be the solution to re-addressing the balance of demand for irrigation water and complement the need for more water supplies in Kuwait [4]. The dissolved organics and other contaminants present in the tertiary treated effluent are known or suspected to be detrimental to various reuse applications and still limit full utilization of this valuable resource. What is called quaternary treatment is defined and used as the treatment producing potable water quality to meet unrestricted residential uses and industrial applications requiring ultra-pure water. Membranes of different pore sizes are usually used in this process such as micro-filtration (MF), ultra-filtration (UF), nano-filtration (NF), and hyper-filtration reverse osmosis (RO) in descending pore diameter order. As a general rule, MF is suitable for
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Fig. 6. Water distribution in Kuwait.
the removal of suspended solids, including larger micro-organisms like protozoa and bacteria. UF is required for the removal of viruses and organic macromolecules down to a size of around 20 nm. Smaller organics and multivalent ions may be removed by NF while RO is even suitable for the removal of all dissolved species [5]. However, direct human consumption of this treated effluent may be objectionable for psychological and probably religious reasons, although it can satisfy the potable water requirements and used for human consumption in many parts of the world. The quaternary treated waters are used as direct potable water or recharged to aquifers for storage and then extracted for potable purposes in many parts of the world. A new wastewater treatment and reclamation
plant (WWRP) was built in Sulaibiya, Kuwait, and uses UF and RO to produce potable water quality from the tertiary treated municipal wastewater. The RO removes dissolved salts and harmful contaminants, including bacteria, viruses and chemicals. The WWRP is the largest facility of its kind in the world to use RO and UF membranebased water purification. The plant’s initial daily capacity is 375,000 m³/d and designed for extension to 600,000 m³/d in the future. When fully operational, the facility is predicted to contribute 26% of Kuwait’s overall water demand, reducing the annual demand from non-potable sources from 142 Mm3 to 26 Mm3. The plant water production satisfies the increasing agricultural, industrial and domestic demands and can preserve the natural strategic water resources. The
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plant also reduces the environmental pollution resulting from the direct discharge of secondary– tertiary municipal effluents into the sea. The Ministry of Energy is financing a research project to artificially recharge the Sulaibiya plant output potable quality water in ground water aquifers. The conventional treatment cost is $0.36/m3, which represents less than 10% of the cost of MSF distillate. 3.2. Water demand management Water consumption is really high (600 L/d per capita) in Kuwait because (a) no metering is used, (b) consumers have high income, (c) the water price is very low and is based on a flat rate, (d) weather is hot and dry, and (e) consumers do not recognize the value of water. It takes two sides to conserve water: the consumers and authorities (like municipalities and the Ministry of Electricity and Water). So, there are apparent measures that should be considered such as: 1. Inform, encourage, and teach consumers how to use water efficiently and enforce necessary measures to achieve that; regulate and restrict specific water uses; and develop a public education program. 2. Metering water at the source and points of use, detect leakage to decrease unaccounted for water (up to 50% of water loss are due to leakages in many countries), and repair water lines. 3. Charge fees according to amount used by consumers beyond their basic needs. 4. Invest in appliances, processes, and technologies that can reduce consumed water. 5. Audit water in housing, public buildings, commercial, and industrial sectors should be conducted to know where conservation measure should be applied. 6. Subsidize or supply free of charge only the basic water needs of customers, but beyond that, the exact cost of water should be charged. 7. Apply known conservation methods used in housing such as the use of (a) low-flow plumbing
fixtures, use of pressure reduction devices, and low-flow shower-heads, (b) toilet displacement devices to reduce the amount of water used by flush, e.g., placing a milk jug filled with water in the tank., (c) replacing conventional toilets, which use 15–20 L/flush with low flush toilets which use 6 L/flush or less, (d) stopping the use of hoses in car washing, and sweeping side walks and driveways. Water users (households, agriculture and industry) should be consciously oriented to save water. 3.3. Fuel energy consumed by cars Cars are another big fuel consumer. Cars are also a big source of air pollution due to fuel burning, which emits CO2, SO2, NOx, CO, hydrocarbons (fuel vapor when exposed to air), and particulates. Table 4 gives the fuel consumption and the pollutant of a car driven an average 15,000 km/y. There are about one million cars registered in Kuwait and a good fraction of them are big cars. For an average car traveling 15,000 km/y, the emission rates of different gases and consumption in g/l and in kg/y are shown in Table 4. The main reason of high fuel consumption by cars compared to other transportation methods is the low number occupants traveling in each car, as shown in Table 5. There are simple rules to decrease the energy consumption and polluted gases emitted by cars and these include: 1. Use efficient and small cars and keep them under best performance conditions. The government should tax large cars. 2. Use car pools with as many partners as possible. 3. Use public transportation like buses or trains. Kuwait has a good public bus system, but it needs more lines, and a train system. The public transportation system including railways and more bus lines should be expanded by the government.
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Table 4 Emission rates of different gases, and consumption in g/l and in kg/y for an average car traveling 15,000 km/y Component
Emission rate and consumed fuel
Calculation
Total annual pollution emitted and fuel consumed
CO NOx CO2 Gasoline
1.68 0.83 256.90 0.1044
25.15 12.49 3853.45 1566.00
kg/y kg/y kg/y L/y
(g/km)×15,000/1,000 (g/km)×15,000/1,000 (g/km)×15,000/1,000 (l/km)×15,000
Table 5 Energy use in MJ per passenger per mile Transport mode
Number of people carried
Energy use (MJ) per passenger mile
Large car (in town) Large car (open road) Small car (in town) Small car (open road) Train (intercity) Train (commuter) Bus Motorcycle Walking Cycling
1.5 1.5 1.5 1.5 50% full 65% full 50% full 1 1 1
6.8 3.9 3.8 2.7 2.1 0.9 1 4.3 0.4 0.1
3.4. Electric power demand management Air conditioning systems in Kuwait consume about 75% of the electric energy generated at peak load time. Reduction of the energy consumed by A/C will reduce the peak load. In Kuwait, the price of electric energy is highly subsidized (93%). It is sold for 2 fils/kWh, while its production cost is 30 fils/kWh. Due to this low price, there are no real public incentives to save energy. Therefore, the government should enforce saving energy by imposing codes or regulations, especially in the A/C area. The only measures imposed by the Ministry of Energy to save energy are:
1. Setting maximum wattage/m2 of conditioned spaces when current is connected to buildings; 2. The use of water cooled condensers in A/C units with capacity higher than 500 t (1750 kW) cooling. 3. Minimum coefficient of performance (COP) is imposed on imported A/C equipment, such as 1.75 for air-cooled and 2.5 for watercooled condensers, including all electric-driven devices. In addition, non-mandatory guidelines are applied, and include the following: 1. Walls and roofs should have minimum resistance of 1.76 m2 °C/W and 2.54 m2 °C/W, respectively. 2. Maximum glazing to wall area ratio is 10% for ordinary glass and 15% for double-glazing. 3. Maximum allowable outside air is one air change/hour. In fact, these measures, except the insulation required for walls and roofs, are far from ensuring efficient A/C systems. The ASHRAE issued new standards providing minimum requirements for saving energy in new buildings or new additions to existing buildings in USA. At least these standards should be adopted and enforced in Kuwait. The ASHRAE standards (ASHRAE/ INSA 90 2002) are stricter than the Ministry of Energy and Water (MEW) guidelines proposed in the early 1980s. An example is given here [6]: the cooling load for a typical 265 m2 area villa was
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calculated as 46.82 kW per villa. The rate of consumed electric energy is 26.75 kW, based on the mandated 1.75 COP by the MEW. When the walls and roof were insulated according to the ASHRAE code, the cooling load for the 265 m2 was reduced to 30 kW. The minimum COP of air-cooled package and split A/C systems by ASHRAE standards is 2.93 at US-rated conditions. This can be corrected to 2.5 for Kuwaiti conditions (compared with 1.75 suggested by MEW). For the considered villa of 265 m2 area, the use of an efficient A/C system with minimum COP of 2.5 gives the consumed electric energy equal to only 12 kW when the load is decreased to 30 kW. This shows that by raising the insulation of walls and roofs and using more efficient A/C system, the electric consumption of the A/C was reduced to less than half. References [1] M.A. Darwish, F. Al-Awadi and A.M. Darwish, Energy and water in Kuwait: A sustainability viewpoint, Part I, Desalination, 225 (2008) 341–355. [2] M.A. Darwish, S. Al-Otaibi and K. Al-Shaigi, Suggested modifications of power-desalting plants in Kuwait, Desalination, 216 (2007) 222–231. [3] M.A. Darwish and N. Al Najem, The water problem in Kuwait, Desalination, 177 (2005) 167–177. [4] M. Abdel-Jawad, S. Ebrahim, M. Al-Tabtabaei and S. Al-Shammari, Advanced technologies for municipal wastewater purification: technical and economic assessment, Desalination, 124 (1999) 251–261. [5] T. Wintgens, T. Melin, A. Schafer, S. Khan, M. Muston, D. Bixio and C. Thoeye, The role of membrane processes in municipal wastewater reclamation and reuse, Desalination, 187 (2005) 1–11.
Appendix Table A1 Main data for the CPDP (with and without steam extracted to desalting unit) in Doha West, Kuwait Nominal capacity, 300 MW
Steam No steam extracted to extracted desalters to desalters
Output steam flow rate (kg/s) Throttle pressure (bar) Throttle temperature (C) Reheat pressure (bar) Reheat temperature (C) Final feed temperature (C) Condenser pressure (bar) Extracted steam flow rate to desalting unit (kg/s) Extracted steam pressure (bar) Extracted steam temperature (C) Enthalpy of steam inlet to desalting unit (kJ/kg) Enthalpy of condensate leaving the desalting unit (kJ/kg) Heat flow rate (kJ/s)
297.6
261.037
139 535 38.4 535 246.4 0.0637 76.72
139 535 36.7 485 246.1 0.0851
4.74 249.5 2960.9 406.1
195,997