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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