On the reduction of desalting energy and its cost in Kuwait

All seawater desalting processes, multi-stage flash (MSF), multi-effect boiling (MEB), mechanical vapor compression (MVC) and seawater reverse osmosis (SWRO) consume significant amounts of energy. The recent increase of fuel oil cost raises the cost of energy consumed for desalting water and the final water cost, and creates more interest in using more energy efficient desalting systems. The most used desalting systems by distillation (MSF and MEB) are usually combined with power plants in what is called co-generation power desalting plants, CPDP. Fuel is supplied to the CPDP to produce both desalted water D and power W, and the fuel cost is shared between D and W. Exergy analysis and equivalent work are among the methods used to determine the fuel energy charged to each product. When desalting systems, such as SWRO and MVC, are not combined with a power plant, the fuel energy can be directly determined from its electrical power consumption. In this paper, the fuel energy cost charged to desalting seawater in the presently used CPDP in Kuwait is calculated based on exergy analysis. The MSF, known by its high energy consumption, is the only desalting method used in Kuwait. The MSF units consume 258 kJ/kg thermal energy by steam supplied to the brine heater BH, 16 kJ/kg by steam supplied to steam ejectors, and 4 kWh/m3 mechanical energy for pumping. These MSF units are operated either by: (1) Steam extracted from extraction/condensing steam turbines EC/ST as in as in Doha West, Azzour, and Sabbiya CPDP. This practice is used in most Gulf area. (2) Steam supplied directly from boilers as occurred in single purpose desalting plants as Al Shuwaikh plant; or in winter time when no steam turbines are in operation in the CPDP to supply steam to the desalting units. The CPDP have limited water to power production ratio. While they can cope with the increase of power demand, it cannot satisfy the water demand, which is increasing with higher pace than the power demand. The case of steam CPDP used in Kuwait is presented in this paper as a reference plant to evaluate the amount of fuel energy consumed to desalt water in MJ/m3, its cost in $/m3. The resulted high fuel cost calls for some modifications in the reference CPDP to lower the energy cost, and to increase its water to power ratio. The

The exergy analysis is used to allocate the cost of fuel used in the CPDP between the plant power and desalted water outputs. A Comparison between all the cases considered in this study is summarized in Table 5. (1) The extra fuel energy added to the steam generator in order for the plant to produce the 196 MW heat required for the desalination is less than 60% of Qd added to the desalting units. (2) Although the specific heat to the desalting system is the same, 258 MJ/m3, and almost the same gain ratio for cases 1 (MSF with steam extracted from turbine), 2 (MEB with steam extracted from ABPST turbine), and 4 (MSF with steam direct from CPDP steam generator), the fuel energy charged to these cases 209.9, 115.5, 379, 327.4 MJ/m3, and fuel cost/m3 distillate $2.21, $1.21, and $3.99 respectively. This shows that the terms used to rate these desalting units do not give realistic evaluation of fuel cost. (3) The use of more exergy efficient MEB (compared to MSF) system saves more than 40% of the fuel cost due its supply with low temperature steam (compared to MSF). (4) The fuel energy charged for the mechanically driven SWRO and MVC systems are significantly less than the MSF and MEB in all cases, especially when the high efficient gas/steam combined cycle is used for power production. (5) According to this exergy analysis, supplying steam directly from steam generator (or boiler) should be avoided as it raises the cost of fuel compared to steam extracted from turbines, and finally (6) The rise of fuel energy calls for the use of more energy efficient desalting systems. The exergy analysis provides a rational method of evaluating the fuel energy charged to produce desalted water and electric power in CPDP. Nomenclature a BH CPDP D E e GR h HHV HPT IPT LPT M MEB MSF Qd Qf Qfw Qfd RO s specific exergy (kJ/kg) brine heaters of the MSF units cogeneration power desalting plant desalted water (kg/s) extensive exergy (KW) or (MW) effectiveness or second law efficiency gain ratio (D/Sd) enthalpy (kJ/kg) fuel high heating value (kJ/kg) high pressure turbine intermediate pressure turbine low pressure turbine mass flow rate (kg/s) multi effect boiling multi stage flash desalting units heat added to the BH of the MSF units, or MEB first effect fuel energy added to CPDP fuel energy added to produce work (W) fuel energy added to produce distillate (D) reverse osmosis specific entropy (kJ/kg K) Sd SWRO T UF steam supplied to the BH of MSF units or first effect of MEB units seawater desalting system temperature utilization factor (W + Qd)/Qf Subscripts: numbers for state points shown in Fig. 2. b f fw fd s si r w we o boiler fuel fuel for work fuel for desalting steam steam inlet reheated steam work water exit surrounding temperature