Chemical Engineering January 2013 - 36

Cover Story
TABLE 3. COMPARISON OF TYPICAL CAPITAL COSTS FOR VARIOUS
POWER CYCLES
Conversion Technology Typical Sources of Waste Heat Capital Cost
system. The pressure of steam generated
will depend on the fluegas (hot
stream) temperature. Steam pressure
levels can be optimized with the available
fluegas temperatures and based
on the plant's steam balance. Steam
generation has the advantage that
piping costs may be less due to proximity
of steam headers in the plant;
its disadvantage is that steam header
pressures at a petroleum refinery, for
example, are usually fixed, and hence,
cannot maximize the amount of possible
heat recovery. Generation of high
pressure steam is preferred as it can
be used for power generation. However,
HP steam generation will lead
to lower heat recovery from fluegas or
another hot stream. A more costly and
efficient system will use steam generation
followed by air preheating.
In addition to steam generation,
economizer coils can be added to heat
water or intermediate heat transfer
fluids. Saturated steam generated in
the steam generators can be super
heated by recovering heat from fluegases
or hot streams. A hot oil circuit
can be installed to supply heat to multiple
locations in the process unit. A
hot oil system can maximize the heat
recovery, but requires additional capital
investment.
Gas turbine. The exhaust gases from
a GT (with and without duct firing)
can be used for steam generation in
HRSG at multiple pressure levels; or,
it can be used to heat process streams.
In some instances, the hot turbine exhaust
is used as combustion air for a
fired heater in the plant where a GT is
located. Waste heat of HRSG exhaust
can be used to produce chilled water
using an absorption chiller or jet refrigeration
to cool GT inlet air, and hence
increase power output of the GT. It can
also be used for organic Rankine or Kalina
cycles (discussed in the next section)
to produce power. The optimum
choice of heat recovery method will depend
on many factors, such as process
heating requirements, available space,
fluegas quantity, quality, refinery steam
balance and payback criteria.
Heat recovery for power
All forms of energy, including work,
can be fully converted into heat, but
the converse is not generally true. As
Traditional steam cycle Exhaust from gas turbines, reciprocating
engines, incinerators
and furnaces
Kalina cycle
Gas turbine exhaust and boiler
exhaust
Organic Rankine cycle Gas turbine exhaust, boiler exhaust
and heated water
per the second law of thermodynamics,
only a portion of the heat from a heatwork
cycle - such as a steam power
plant - can be converted to work. The
remaining heat must be rejected as
heat to a sink of lower temperature
(atmosphere, for instance). For any
process converting heat energy to mechanical
energy, the Carnot efficiency
is the theoretical maximum.
Organic Rankine cycle (ORC). This
can work with waste heat streams in
the lower-temperature range of 80 to
400°C [35] to generate electricity. An
ORC engine is similar to a steam Rankine
engine, except that it uses a lowerboiling-point
organic fluid, instead of
steam, as the working fluid. The working
fluid is vaporized in the evaporator
using waste heat, and the resulting
high pressure vapor is expanded in a
turbine to generate power. Low pressure
vapor from the turbine is condensed
in the condenser using cooling
water or air. Finally, condensed working
fluid is pumped to high pressure to
the evaporator, to complete the cycle.
An economizer is generally added
to reduce condenser cooling load and
improve ORC efficiency, as illustrated
in Figure 11. This cycle has the highest
temperature at the evaporator and
the lowest temperature at the condenser.
In ORC, working fluids having
higher vapor pressure than water
are used. So, operating pressures and
temperatures of ORC are lower than
those of the Rankine cycle.
For working fluids with lower boiling
points, the turbine inlet pressure
can be higher and the circulating mass
flow is lower (minimization of operating
costs), thereby requiring a smaller
size turbine. This results in no condensation
during expansion in the turbines,
which ensures longer life spans
for turbine blades, and therefore super
heating of the fluid is not required before
expansion in the turbine.
Thermodynamic properties of working
fluids affect the system efficiencies.
An ORC working fluid should have a
36 CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2013
$1,100-1,400/kW
$1,100-1,500/kW
$1,500-3,500/kW
mainly positive or isentropic saturation
vapor curve, high vapor density,
high critical temperature and high
heat stability. Liu and others [30] presented
the effect of working fluids on
ORC performance for WHR. Fluids
used in ORC include propane, butanes,
CFCs, freon, n-pentane, iso-pentane,
hexane, ammonia, R245fa, octamethylcyclotetrasiloxane
(D4) and many
other proprietary fluids. Saleh and
others [40] presented the performance
of ORC for various working fluids for
a maximum evaporator temperature
of 100°C. A screening study of several
working fluids based on power production
capability and equipment size requirements
was presented by Lakew
and Bolland [28]. It shows that R227ea
gives the highest power for a heatsource
temperature range of 80-160°C
and R245fa produces the highest in
the range of 160-200°C. Wei and others
[44] studied the performance and
optimization of ORC for WHR.
The extent of heat recovery can be
calculated from exergy (available energy)
of the waste heat stream. For estimating
the electric power recovered,
the following formula can be used:
Electrical power, kW = ηe × ηcarnot ×
WH = ηo × WH
(5)
Where ηe is exergy efficiency; ηcarnot=
Carnot efficiency = 1 - (cold source
temperature, K / waste heat stream
temperature, K, ηo = ORC efficiency
and WH is the waste heat in kW. For
a quick estimation of power, one ORC
supplier, Cryostar (www.cryostar.com/
web/heat-conversion.php, accessed in
January 2012) indicates a value of 0.5
for ηe. Labrecque and Boulama [27]
stated that, for waste heat to useful
work conversion, exergy efficiency
as high as 70% is conceivable. Bourji
and others [10] proposed a correlation
for approximately estimating
ORC power generation from fluegas
temperatures between 350 and 500°F
with ambient temperatures varying
between 50 and 100°F. They also es
http://www.cryostar.com/ http://WWW.CHE.COM
Chemical Engineering January 2013

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