Written Presentation Power and Desalination Integrated Systems : Today s Trends

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1 Written Presentation Power and Desalination Integrated Systems : Today s Trends Abstract Jacques Andrianne Tractebel Energy Engineering 7 avenue Ariane, B-1200 Brussels, Belgium Due to the fuel cost increase in the past years, global efficiency of power & desalination systems has become more and more a key issue. While substantial progress has been made in the efficiency of power systems, desalination concepts have evolved dramatically in the recent years: desalination thermal process is moving from MSF to MED, hybrids involving both thermal & membrane processes are more and more implemented, various integrated desalination systems are studied and more generally speaking the optimisation of power and desalination integrated systems is one of the today s challenges, the desalination community is facing. The presentation will focus on the various aspects of the power and desalination integrated systems as well as on the nowadays trends in the field. Introduction While the electricity demand will steadily increase world-wide in the next years, the desalinated water demand will increase sharply specially in the deserted areas. The challenge is to be able to meet such future demand, minimizing the electricity and water production costs. To achieve this goal, more and more efficient power generation systems have been designed and implemented by the power industry. Reducing the usage of energy contributes to the respect of the environment lowering negative local and global effects. Similarly, the desalination industry has made substantial progress in this field. The improvements cover various aspects such as increasing the quality and reliability of individual processes, designing more efficient processes, integrating some of them, creating hybrids and revamping or using existing facilities. The global gain could be further increased by combining the production of desalinated water and electricity using optimised power and desalination integrated systems. More efficient power generation systems At a time when the oil price was low and the basic need was desalinated water, it was not uncommon to have a thermal desalination process fed by steam produced by a conventional oil-fired boiler. With the increasing need of electricity and realizing the virtues of cogeneration processes, thermal desalination plants have been associated with power generation systems. 144

2 Thermal desalination processes have been coupled to back pressure steam turbines, to extraction-condensing steam turbines, but also gas turbines with heat recovery steam generator. The selection of the associated power generation system was depending on various characteristics such as electricity on water production ratio, flexibility in the electricity production keeping the water production steady, global efficiency, Fuel cost increase and environmental impact concerns encouraged the development of power systems based on gas turbines. Gas turbine dimensions were increased and thus air mass flow capacity. Further development includes refinements in the aerodynamic design without major changes in the dimensions. Output was again increased by the introduction of blade cooling as well as heat-resistant materials and coatings. At the same time key developments have been achieved in the heat recovery steam generator technology : higher efficiency is obtained by the use of higher pressure, oncethrough boiler technology as well as by optimised multi-pressure designs. Recent deregulation has accelerated the growth of Independent Power Producers (IPP). These power producers have provoked a growth in new power production as well as a shift from conventional steam plants to gas turbine plants including combined cycles. SOURCE : ALSTOM POWER The shift towards combined cycles is justified by economic and environmental factors : the net efficiency of combined cycle power plant (55 %) is much higher than with conventional steam plants (35 % to 40 %); 145

3 the combined cycle need a shorter time to be built and started up, thereby reducing the capital cost and risk to plant developers; gas fired combined cycles require much lower investment cost per MW; modern gas turbines are known for very low environmental impact. Nuclear power generation is also an interesting alternative although, up to now, jeopardized by socio-political obstacles. Also renewable power technologies have been developed and implemented to meet the green electricity requirements, but they still need to be more cost effective. Increasing quality and reliability of individual desalination process The MSF process capacities have increased from the small 1 MIGD units of the early sixties to the present day 12.6 MIGD units installed at Al Taweelah B in Abu Dhabi. While the plant design is still very similar to the first units built, great improvements have been made in the selection of materials to give reliable operation without suffering from the corrosion problems which were evident in early plants. The formation of scale which was not fully understood initially can now be controlled by the addition of chemicals. The use of materials such as titanium which gives good heat transfer properties with high corrosion resistance has further enhanced the process. Another limiting factor to increase the plant size was the availability of reliable high capacity pumps. Such pumps are now available and in use. Early RO plants were developed for the treatment of brackish water and the membranes used were not suitable for use on sea water. Since the early days the membranes have been improved and they are now in use in sea water RO (SWRO) plants in the Middle East and elsewhere. The high salinity of the sea water in the Gulf region has now been better mastered and potable water is now being produced from Gulf sea water. The RO process requires good quality feedwater to be successful. The membranes are very sensitive to suspended solids, certain chemicals, pollution and biological fouling. In order to maintain the membranes in good order and to give them reasonable life, great care is required by the operators to ensure that water chemistry requirements of the plant are adhered to. Substantial progress has been made by using deep sea water intakes, improved pretreatment and better skill operators. Designing more efficient desalination processes For large units plants (unit size > 5 MIGD) the MSF process is still implemented, being well proven and reliable. Considering the process aspects of large MSF units (size in the range of 10 to 15 MIGD), there is still some room for optimization in various fields such as fouling factors, weir load, seawater recirculation systems, vacuum system, 146

4 For smaller plants (unit size up to 5 MIGD), MED process has made substantial progress. It allows good heat transfer rates with acceptable scaling which can be further improved by the addition of anti scale chemicals and the control of temperatures below 70 C. In order to improve thermal efficiency and reduce the number of cells required, vapour compression is used. This is where some of the vapour produced in the last cell is recompressed and introduced back into the first cell where its pressure energy is released. The use of vapour recompression has the effect of reducing heat energy consumption. Due to the limited number of cells required in a MED-TVC plant, considerable savings can be made on capital expenditure compared to a MSF plant. MED plants require lower auxiliaries consumption and in the future, due to recent development, the MED will be more and more implemented even for larger size. As far as SWRO is concerned, power is required to drive the high pressure feed pump which is used to raise the sea water feed above its osmotic pressure (around 70 bar) at which pressure it is delivered to the inlet of the membrane. Product water is produced in the ratio of approximately 1/3 of the feedwater. The balance of the feedwater is rejected as brine and since this rejected brine is still at relatively high pressure, an energy recovery turbine can be fitted to recycle this energy and so reduce the amount of power required for the process. Specific power consumption has been brought down substantially by using more and more efficient energy recovery devices, such as hydro turbo charger and pressure exchanger system. Creation of desalination hybrids More and more locations in the world are in real need of water, but not necessarily in need of electricity. In this case, an optimal arrangement will involve a combination of desalination thermal and membrane processes. Natural Gas RO 30 MIGD ~ LM MWe 30 C Gas Turbine Steam at 142 C 3 bara Heat Recovery Boiler 62 t/h GOR=8 MED 2.6 MIGD Figure 2 Typically, a gas turbine LM6000 PD generating 33 MW at 30 C is able to feed in electricity a RO plant producing 30 MIGD. To benefit from the heat rejected by the gas turbine, a heat recovery boiler is installed: without any post-combustion, 62 t/h steam at 3 bar and 142 C can be produced. This amount of steam is able to feed a MED unit of 2.6 MIGD with a GOR of 8. Hence the total water production is close to 33 MIGD. 147

5 Another simulation has been done with a LM gas turbine: the gas turbine is producing about 23 MW and can feed a 20 MIGD RO plant; with post-combustion in the heat recovery boiler 102 t/h of steam can be produced feeding a 4 MIGD MED unit with a GOR of 8. Natural Gas RO 20 MIGD ~ LM MWe 30 C Gas Turbine Natural Gas Heat Recovery Boiler Steam at 142 C 3 bara 102 t/h GOR=8 MED 4 MIGD Figure 3 This creation of desalination hybrids is improving the global efficiency which can be further enhanced by the following synergy : the use of heat rejected from the thermal desalination plant will result in optimum and near-constant feedwater temperature to the RO plant. Another advantage is linked to the product water quality : whereas both MSF and MED processes produce distillate at around 10 to 25 ppm TDS, a SWRO plant produces product at around 450 ppm TDS and thus blending could occur allowing reduction in the potabilisation and remineralisation costs. In this configuration, when the gas turbine is not available or under maintenance, electricity needed to feed the RO plant is taken from the grid. In other countries, not only water is required, but also there is a need of electricity : similar desalination hybrids could then be implemented with more electricity production. It is worth to note that in the Gulf countries, while the water demand (production) is rather steady all over the year, electricity demand (production) varies drastically from summer to winter in the ratio 2.5 to 1. During the winter time, thermal desalination is partly fed from HP steam through reducing stations with very poor efficiency and so there is room to produce more electricity passing this steam through the turbine, this electricity feeding a RO plant. Typically, the desalination plant that has been awarded recently in Fujairah Emirate is an hybrid composed of 60 MIGD MSF process and 40 MIGD RO process. The RO plant provides an electrical base load for operation during periods of steady water demand but low power demand. In this particular case, several savings can be achieved : energy recovery turbines are not required in the RO plant; in the seawater intake and outfall infrastructures; in the potabilisation equipment by blending the distillate and the permeate. 148

6 Another interesting application of the desalination hybrids is softening the seawater through nanofiltration before entering a thermal desalination process. This configuration will reduce scaling and thus will allow higher top brine temperature (TBT), which will contribute to increase the distillate production. In the future, use of UF/NF upstream of RO and thermal desalination plant has also been envisaged to further improve the hybrid plant performance. Revamping of existing facilities Repowering and retrofitting of existing power/desalination stations in the Middle East with increase of the seawater desalination capacity as well as the electricity production capacity could be foreseen as an attractive way of meeting the fast increasing power and water demand in the area. Most of the plants in the Middle East are based either on conventional boilers with steam turbines for power generation feeding multistage flash (MSF) distillation for seawater desalination or gas turbines and associated heat recovery boilers feeding MSF distillation. On the power side, provided it is worth to increase the power production, i.e. real increase in the demand, network capacity easy to cope with such demand, selling price of electricity justifying the investment, repowering leading to combined cycles is advisable : this can be achieved either by replacing existing boilers feeding steam turbines by new gas turbines and heat recovery boilers or adding steam turbine(s) to existing gas turbines and heat recovery boilers. On the water side, various possibilities can be considered for retrofitting : keeping the existing MSF units and adding a RO plant taking its feedwater from the cooling water at the outlet of the power plant condensers and the MSF plant heat rejection sections; dismantling the MSF units and adding a RO plant taking its feedwater from the MSF plant sea water intake and the cooling water at the outlet of the power plant condenser; 149

7 replacing old MSF units with efficient thermal desalination processes such as MED or high temperature once-through MSF, and adding a RO plant taking its feedwater from the cooling water at the outlet of the power plant condensers and the thermal desalination plant heat rejection sections. All the above repowering and retrofitting will lead to an improved global efficiency and will reduce both the electricity and water costs. The choice of the type of repowering and retrofitting will depend of the specific conditions prevailing on site: existing facilities, real demand increase, land available, optimal ratio between thermal desalination and RO, some limitation of the RO process with very high TDS. Optimization of power and desalination integrated systems From the previous sections, it can be seen that on a global point of view, optimization of power and desalination integrated systems leads to combined cycle feeding a desalination hybrid consisting in both thermal and membrane desalination. However, the optimal solution has to be assessed carefully for each location, as it depends on the specific conditions prevailing on the site : existing facilities, power and water demand increase, land availability, raw water quality (seawater TDS), quality of water to be produced, ratio between power and water production, ratio between thermal and membrane desalination. Hence, in certain locations, where there is no electricity requirement, just enough electricity could be produced to feed a combination of thermal and membrane desalination. In other locations, only a RO plant will be connected to the grid. Repowering and retrofitting could also be a solution to reach the optimisation of such systems. In specific cases, there could be alternatives to combined cycles to generate electricity: electricity produced by solar energy can feed a RO plant, nuclear power plants can produce electricity and steam to feed desalination hybrids. In India, the Kalpakkam nuclear power station is feeding a MSF-RO plant, the two qualities of fresh water produced by this hybrid plant being utilized for both industrial and potable water purpose. In power and desalination integrated systems, great case should be brought to the optimisation of all the links between the various processes : pipe connections, cable connections between power generation and desalination systems, links to seawater intake and outfall, links to the grid, links between the thermal and membrane processes in hybrid plants. So, optimisation of power and desalination integrated systems has no general solution, but requires an in-depth analysis of each case in order to implement the most appropriate combination of equipment. References Dr. Becker, B.; Dr. Hoffmann, S.; Dr. Streb, H. Operating Experience with V94.3A Gas Turbines, Paper presented at PowerGen Gottung, E.J.; Mastronarde, T.P. Heat Recovery Steam Generator Developments for the Global Market, Paper presented at PowerGen Bastianini, M.; Bracaloni, N.; Gabiccini, S. Enel Produzione Approach To New Combined Cycle Power Plants, Paper presented at PowerGen