A case study of a power/water cogeneration plant was used to compare plant design characteristics with actual plant performance data. The gas turbine cycle used in the generating company is a constant flow cycle with a constant addition of heat energy. The Brayton power cycle is the ideal thermodynamic cycle that approximates the operation of a gas turbine cycle in the case study. Figure 45.1 depicts the open, or simple cycle, characteristic of a typical gas turbine installed in the case study plant. The gas cycle is composed of a compact set formed by a compressor, a combustion chamber and a gas turbine connected to an electric generator.
In Fig. 45.1, air is compressed from point 1 to point 2 via an axial flow compressor. Fuel and an ignition source are subsequently added resulting in combustion and ultimately the addition of heat to the system between points 2 and 3. Work is then extracted by the turbine between points 3 and 4 due to the expansion of hot combustion gases. Since the work produced by the turbine exceeds the work consumed by the compressor between points 1 and 2, useful work is produced and is used to turn an electrical generator. The hot exhaust gases from the power generation gas turbine section are used in the waste heat recovery boiler section as a heat source to generate steam required in the distillers. Waste heat recovery boilers operate under two conditions: the unfired condition and
the fired one. For normal operating conditions, 160 t/h dry saturated steam is supplied at 15 bars, while the feed water temperature is maintained at 140 °C. Auxiliary boilers are also available to produce steam of similar properties to that produced by the waste heat recovery boilers. However, the steam capacity of the auxiliary boilers is limited to 80 t/h when supplied with feed water at 140 °C. The auxiliary boiler efficiency ranges from 85 to 86 %. A simplified schematic for the multi-stage flash (MSF) desalination process is shown in Fig. 45.2. The MSF plant consists of the following major sections: Distillation, heat input section, heat recovery section and the heat rejection section. The MSF distillation process was designed to produce a total of 216,000 m3/day of distilled water with a top brine temperature of 91 °C or 270,000 m3/day with a top brine temperature of 112 °C.
The heat input section consists of the brine heater, where thermal energy is supplied. In the heat recovery section the heat of condensation is transferred to the brine, and thus pre-heats the recirculated brine in stages 1-14. In the heat rejection section, the heat of condensation is partly transferred to the make-up stream and mostly to sea water cooling stream in stages 15 and 16. The MSF has two operating conditions; one with a top brine temperature of 91 °C when polyphosphate type – chemicals are used for scale inhibition and the other with a top brine temperature of 112 °C when Belgard-EV is used for scale inhibition. The design parameters for the MSF are shown in Table 45.1.
Fig. 45.2 Simplified schematic for the multi-stage flash (MSF) desalination process