Heat Recovery in Kalina Cycle V.Ganapathy

The Kalina cycle is a new concept in heat recovery and power generation,which uses a mixture of 70% ammonia-30% water as the working fluid with the potential of significant efficiency gains over the conventional Rankaine cycle.Basically this concept is suitable for medium to low gas temperature heat recovery systems with gas inlet temperatures in the range of 400 to 1000 F,offering more gains (over Rankaine cycle) as the gas temperature decreases.
Gas turbine based combined cycles using this concept have 2-3 % higher efficiency over multi-pressure combined cycle plants using steam/water as the working fluid. In low gas temperature heat recovery systems such as diesel engine exhaust or fired heater exhaust,the energy recovered from the hot gas stream is more significant and Kalina cycle output increases by 20-30 %.The main reason for the improvement is that the boiling of ammonia-water mixture occurs over a range of temperatures,unlike steam and hence the amount of energy recovered from the gas stream is much higher. See Figure below,where a 550 F gas temperature source is shown with say a cold end fluid temperature of 100 F. 70 % ammonia-water mixture at 500 psia by virtue of its varying boiling point,is able to "match" or run parallel to the gas temperature line while recovering energy and hence the exit gas temperature can be as low as 200 F.The steam-water mixture at 500 psia,on the other hand,due to pinch,approach point limitations and a constant boiling point of 467 F,cannot cool the gases below about 500 F.Only about 15-20 % of the energy is recovered,compared to 100% in Kalina cycle.This can also be easily be seen using the HRSG simulation software for gas turbine-steam systems developed by the author(see my Homepage). Hence a lot of energy is wasted. By decreasing the steam pressure,more energy could be recovered;however,the average fluid temperature decreases,thus lowering the Rankaine cycle efficiency.Multiple pressure systems could recover more energy but add to the complexity of the system and cost.Note however as the inlet gas temperature increases,say to 1000 F,the difference in the amount of energy recovered between steam-water system and ammonia-water system reduces significantly.This can be seen by analyzing the gas temperature profiles for the two cases.The exit gas temperatures for the two cases will be comparable and not so wide apart as in the 550 F case.

The condensation of ammonia-water also  occurs over a range of temperatures and hence permits additional heat recovery in the condensation system,unlike Rankaine cycle,where the low end temperature(affected by ambient conditions) limits the condenser back pressure and power output of system.If the cooling water temperature is say 100 F,less power is generated by the steam turbine compared to say 40 F cooling water.The condenser pressure can be much higher in Kalina cycle,and the cooling water temperatures do not impact the power output of the turbine as in Rankaine cycle.Thermo-physical properties of ammonia-water mixture can also be altered by changing the concentration of ammonia.This helps to recover energy in the condensation system.Modifications to the condensing system are also possible by varying the ammonia concentration and thus more energy can be recovered from the exhaust gases.
Expansion in turbine gives a saturated vapor in Kalina cycle compared to wet steam in Rankaine cycle,which requires protection of blades in the last few stages.Also due to the higher pressure of vapor and lower specifc volume,the exhaust system size can be smaller compared to steam.For example the specific volume of a 70% ammonia-water mixture exhausting from a turbine at its dew point of 240 F is 5.23 ft3/lb,while steam at its condensing temperature of 70 F(sat pres=0.36 psia) has 868  ft3/lb.Thus the equipment size can be smaller with Kalina system.
Conventional equipment such as steam turbines and HRSGs can be used in Kalina cycle.The molecular weight of ammonia and water are similar,17 and 18.
Since the boiling point is varying,once through type HRSGs are used in kalina systems.Carbon steel tubes are adequete. Extended surfaces may be used if the gas stream is clean. For more information on Kalina cycle contact Exergy Corp,California.

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