Kalina Cycle

Kalina Cycle

The Kalina cycle uses a mixture of 70% ammonia and 30% water as the working fluid instead of water, which has a constant boiling point. The problem with constant boiling temperature of water–steam is that with a minimum pinch point, the amount of energy recovered from a low-temperature heat sources such as diesel engine or cement plant will be very small, and hence, the heat recovery system will be uneconomical. Figure 4.35 shows the gas–fluid temperatures for a 500 psia steam and Kalina system with an inlet gas temperature of 550°F. With a, say, 30°F pinch and 10°F approach, the exit gas temperature will be about 474°F. This is a thermodynamic restriction, and more surface areas can proba- bly lower the pinch point slightly but cannot bring down the exit gas temperature to much below 450°F. However, with the Kalina cycle, the same 500°F inlet temperature permits the gases to be cooled to 200°F. This is feasible as the ammonia – water mixture boils over

a range of temperatures, and the boiling curve follows the gas temperature line while the pinch point restricts the energy that can be recovered with steam. Since the boiling occurs over a range of temperatures, it is possible to recover more energy from the same low inlet gas temperature. The amount of energy recovered is (550 − 200)/(550 − 474) = 4.6 times

224 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

a c – gas temperature a – temperature with varying boiling point b b – temperature with constant boiling point

erature, °F mp

200 Te

100 70/30 ammonia–water at Water–steam at 500 psia

Gas–fluid temperature profile with steam and ammonia–water mixture. more. A key feature of Kalina cycle is the ability to vary the ammonia–water concentration

throughout the power plant to optimize energy conversion and to add heat recuperative stages for increased efficiency providing a richer concentration throughout the heat acqui- sition stage and leaner composition in the low-pressure condenser system.

In a typical steam-based Rankine cycle, the energy loss associated with the condenser is large. Also the energy recoverable is much lower when the gas inlet temperatures are low. Hence, Kalina cycle is a good bet when the flue gas inlet temperature to the waste heat boiler is low on the order of 250°C–350°C. As can be seen from Figure 4.35a, the heat is added and rejected at varying temperatures, which reduces these losses. The distillation condensation subsystem (DCSS) changes the concentration of the working fluid enabling the condensation of the vapor from the turbine to occur at lower pressures. The DCSS without any other source of energy brings back the concentration of the mixture back to the 70%–30% level for recovering energy from the waste heat boiler. The heat recovery vapor generator (HRVG) is similar in design to the HRSG. Carbon steel tubes are adequate for the superheater, evaporator, and economizer.

Advantages of Kalina Cycle

1. This is ideal for low inlet gas temperature heat sources in the range of 200°C–300°C. The ammonia–water mixture has a varying boiling and condensing temperature that enables the fluid to extract more energy from the hot gas stream than with a

Waste Heat Boilers 225

steam system that has a constant boiling and condensing temperature; pinch and approach points set the temperature profile and energy recovery in steam sys- tem while the 70–30 ammonia–water mixture can follow the gas temperature pro- file closely as shown in Figure 4.35b. By changing the working fluid to 45%–55% ammonia–water in the condenser system, condensation of the vapor is enabled at a lower pressure that allows more extraction energy in the vapor turbine. The DCSS accomplishes this.

2. The thermophysical properties of the ammonia–water mixture can be changed by changing the ammonia concentration. Thus, even at high ambient temperatures, the cooling system can be effective, unlike in a steam Rankine system where the condenser efficiency drops off as the cooling water temperature or ambient tem- perature increases.

3. The ammonia–water mixture has thermophysical properties that cause mixed fluid temperatures to change without a change in heat content. The temperature of water or ammonia does not change without a change in energy.

4. Water freezes at 32°F while pure ammonia freezes at −108°F. Ammonia–water

solutions have very low freezing temperatures and hence at low ambient tempera- tures can generate more power without raising concerns about freezing.

5. The condensing pressure of the ammonia–water mixture is high, on the order of 1.5–2 bara compared to 0.1 bara in a steam Rankine system, resulting in lower specific volumes of the mixture at the steam turbine exhaust and consequently smaller turbine blades. The expansion ratio is about 10 times smaller. This reduces the cost of the turbine–condenser system. With steam systems, the condenser pressure is already at a low value on the order of 1 psia, and hence, further lower- ing would be expensive and uneconomical.

6. The losses associated with the cooling system are smaller due to the lower con-

densing duty; hence, the cooling system components can be smaller and the envi- ronmental impact lesser.

7. Standard carbon steel material can be used for the HRVG, piping and ammonia-

handling equipment. Only copper and copper alloys are prohibited.

A 3 MW demonstration plant based on Kalina cycle was operating in California for more than

5 years two decades ago (Figure 4.36). In this plant, 14,270 kg/h of ammonia vapor enters the vapor turbine at 110 bara at 515°C and exhausts at 1.45 bara. The main working fluid for the HRVG is the 70–30 ammonia–water mixture while at the condenser it is 42–58 ammonia– water mixture. The leaner fluid has a lower vapor pressure, which allows for additional tur- bine expansion and greater work output. The ability to vary this concentration enables the plant 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. DCSS system consists of demister separator, recuperative heat exchangers, high- and low- pressure condensers, and control system. DCSS enables condensing to be achieved in two stages, first forming an intermediate leaner mixture leaner than 70% and condensing it, then pumping the intermediate mixture to higher pressure, reforming the working mix- ture, and condensing it as shown for admission into the HRVG. In the process of reforming the mixture (back to 70%), additional energy is recovered from the exhaust stream that

226 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Canoga Park HRVG. 1, HRVG; 2, turbine; 3, flash tank; 4, final preheater; 5, HP preheater; 6, second recuperator; 7, vaporizer; 8, HP preheater; 9, first recuperator; 10, LP preheater; 11, HP condenser; 12, LP condenser; 13, cool- ing water; t, throttling device; p, pump.

increases the power output. Calculations show that the power output can be increased by 10–15% in the DCSS compared to the steam-based Rankine cycle.

The HRVG for the Kalina system is simply a once-through steam generator with an inlet for the 70% ammonia liquid mixture, which is converted to vapor at the superheater. The tube-side pressure drop in an HRVG is typically large. About 60 barg is the exit pres- sure, and about 10% of this is the loss in the superheater, evaporator, and economizer. The two-phase pressure drop in the evaporator particularly in the preheater will be high as several bare tube rows are required for recovering energy from the dusty gas compared to the clinker cooler gas stream with its finned tubes. Proper tube-side velocities have to

be chosen to ensure that separation of vapor from liquid does not arise. Plain tubes are used if fouling is a concern as in the preheater flue gas boiler. Finned tubes with a low fin density of 2 to 3 fins/in. is used in the clinker cooler gas stream; the tube-side heat transfer coefficients are lower than steam water exchangers, and hence, using more fin density than

2 to 3 fins/in. will be counterproductive as shown in Appendix E. Figure 4.37 compares the energy recovery potential of Kalina cycle with that of a steam

system for a clinker cooler air stream. It is seen that due to the constant boiling tempera- ture of steam and with 8°C pinch point and 7°C approach, the exit gas temperature is 236°C and energy recovered is only 11 MW. The Kalina cycle on the other hand with its 70–30 ammonia–water mixture with variable boiling temperature follows the gas temperature profile closely as a result of which the exit gas temperature could be dropped to 137° and the energy recovered is 23.5 MW. It is true that the tube-side coefficients with ammonia

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Air flow = 440,000 kg/h Air flow = 440,000 kg/h Pressure = 42 barg

320 Pressure = 42 barg Steam flow = 14,000 kg/h

Vapor flow = 34,500 kg/h 315

Total energy recovered = 11 MW Total energy recovered = 23.5 MW 304

137 Steam HRSG

Kalina HRVG

39 All temperatures in °C

Superheater Evaporator

Economizer

Superheater

Evaporator Economizer 39

FIGURE 4.37

Comparison of energy recovery from steam and ammonia–water mixture. water mixture will be lower than that of steam–water and as a result of which the HRVG

will have more rows deep as large fin density cannot be used for the superheater, evapora- tor, and economizer in the clinker cooler heat recovery system; also due to the presence of dust, large fin density is not recommended. Considering the potential for large heat recovery, the Kalina cycle is attractive when gas inlet temperature gets smaller. For the steam system to recover more energy, one should think of multiple-pressure system and combining of flows in the economizer as shown earlier in another system.