Example of a Kalina System
A 3 MW plant has been in operation in California for more than a decade. In this plant, 31,450 lb/h of ammonia vapor enters the turbine at 1600psia, 960°F and exhausts at 21 psia. The ammonia concentration varies throughout the system. The main working fluid in the HRVG is at 70% concentration, whereas at the condenser it is at 42%. The leaner fluid has a lower vapor pressure, which allows for additional turbine expansion and greater work output. The ability to vary this concentration enables the performance to be varied and improved irrespective of the cooling water temperature.
Following the expansion in the turbine, the vapor is at too low a pressure to be completely condensed at the available coolant temperature. Increasing the pressure would increase the temperature and hence reduce the power output. Here is where the DCSS comes in. The DCSS enables condensing to be achieved in two stages, first forming an intermediate mixture leaner than 70% and condensing it, then pumping the intermediate mixture to higher pressure, reforming the working mixture, and condensing it as shown in Fig. 1.4. In the process of reforming the mixture (back to 70%), additional energy is recovered from the exhaust stream, which increases the power output. Calculations show that the power output can be increased by 10-15% in the DCSS compared to the Rankine system based on a steam-water mixture.
The HRVG for the Kalina cycle is a simple once-through steam generator with an inlet for the 70% ammonia liquid mixture, which is converted into vapor at the other end. The vapor-side pressure drop is large, on the order of hundreds of pounds per square inch due to the two-phase boiling process. Conventional materials such as carbon and alloy steels are adequate for the HRVG components.
Studies have been made on large combined cycle plants using the Kalina cycle concept. Using an ABB 13 E gas turbine, 227 MW can be generated at a heat rate of 6460Btu/kWh (52.8%). This system produces an additional 12.1 MW compared to a two-pressure steam bottoming cycle. Though the cost details are not made available, it is felt that they are comparable on the basis of dollars per kilowatt.
Several variations of the Kalina cycle have been studied. One of the options for power generation cycles is shown in Fig. 1.8. It employs a reheat turbine. A cooling stage is included between the high pressure and intermediate turbines. First the vapor is superheated in the HRVG and expanded in the high pressure stage. Then it is reheated in the HRVG and expanded in the intermediate stage to generate more power. At this point the superheat remaining in the vapor is removed to vaporize a portion of the working fluid, which has been preheated in the economizer section. This additional vapor is then combined with the vapor generated in the HRVG and then superheated. The cooled vapor is then expanded in the low pressure stage. These heat exchanges enable the working fluid to recover more energy from the exhaust gas stream. A 4.5 MW Kalina system is in operation in Japan that uses energy recovered from a municipal incineration heat recovery system, and a 2 MW plant using geothermal energy is in operation in
Figure 1.8 Kalina system to improve energy recovery in a combined cycle plant.
Iceland. It may be noted that as the temperature of the heat source is reduced, the Kalina system offers more efficiency than a steam or organic vapor system.