Chemical Engineering September 2010 - 45

3
4
Engineering Practice
Feature Report
Recover
Waste Heat
From Fluegas
Adding an organic Rankine cycle system
to generate power onsite can help operators
optimize the overall economics of combustionrelated
systems and emissions controls
Ali Bourji, John Barnhart,
Jimmy Winningham, Alan Winstead
WorleyParsons
T
he ever-increasing cost of fuel
and relentless push for environmental
responsibility are constant
factors in the profitability
of many chemical process operations,
and fluctuations in the prices of natural
gas and other fuels can make it
hard to predict future energy costs
and the impact of such fluctuations
on profits. However, the impact of
fluctuating fuel costs, and the regulatory
uncertainty related to managing
emissions can be reduced by increasing
energy efficiency and incorporating
innovation into the design of any
combustion-related system that produces
fluegas.
Numerous industrial processes involving
furnaces, heaters, kilns and
boilers that combust fossil fuels release
large quantities of energy in the
form of waste heat that is contained in
the fluegas. An organic Rankine cycle
(ORC) system can efficiently utilize
this waste heat to generate electricity,
even from relatively low-temperature
fluegas streams.
An ORC resembles a typical Rankine
cycle, but instead of circulating
water as the working fluid, an ORC
uses a refrigerant - typically an organic
fluid such as ethane, propane,
propylene or various name-brand refrigerants,
such as R-245fa (discussed
below). An ORC also operates at lower
temperatures compared to the more
widely used steam-based Rankine
systems. For instance, steam-based
systems typically operate at temperatures
corresponding to low-pressure
steam; that is 250°F or higher. By
comparison, an ORC can efficiently
operate at temperatures below the
boiling point of water, from 212°F to
as low a temperature as desired for
a given application. Thus, the use of
an ORC can be a more cost-effective
method for capturing waste heat from
fluegas compared to the use of a traditional
steam-based Rankine cycle.
This is accomplished by reducing fluegas
temperatures below what is possible
in a steam-based system without
the need for a complex heat exchange
system.
When properly designed, an ORC
system can remove more heat from
a fluegas stream than a steam-based
Rankine system and thus can be effectively
used with cooler fluegas streams
compared to those produced by steambased
Rankine systems.
Meawhile, most fluegas-treatment
methods, such as those involving fluegas
scrubbing, carbon capture, and
carbon sequestration require the fluegas
stream to be cooled prior to its
introduction into the treatment train.
Thus, the addition of an ORC system
can be of great benefit when used in
combination with a downstream fluegas-treatment
system, and can help
E-100
Fluegas
inlet
400°F
1,200
million
std. ft3/d
R
P-100
2
Working fluid: R-245fa
Turbine (T-100) generation, hp
Condenser (E-101) consumption, hp
Pump (P-100) consumption, hp
Total power recovery, hp
Total power recovery, kW
1
1.291 x 104
57.72
1,031
1.182 x 104
8,815
FIGURE 1. This simulation lowsheet
for an ORC system shows the basic
equipment, low streams and power
requirement and recovery based on R245fa
as the working luid. The stream
numbers indicate the order in which the
streams low
to improve the overall economics of
fluegas treatment by generating additional
power from waste heat in the
fluegas stream - heat that would otherwise
simply be lost to cooling water
or to the atmosphere.
The basic ORC scheme
The simple process flow diagram that
is provided in Figure 1 shows a basic
ORC system. The liquid refrigerant
(also called the working fluid) flows
from the surge drum (D-100) to the
pump (P-100) where the fluid is pressurized.
The pressurized fluid is then
sent to the evaporator (E-100). The
evaporator vaporizes the working fluid
by heat exchange with the fluegas
stream. Once vaporized, the working
fluid then enters the turbo-expander/
generator (T-100) where, by process of
expansion, the fluid produces usable
work in the form of electrical energy.
After expansion, the fluid enters the
air-cooled condenser (E-101) where it
is condensed back to liquid form and
returned to the surge drum.
Figure 1 also provides values for
the basic fluegas stream variables
along with a small table that shows
the quantity of power produced by the
turbo-expander/generator train, the
components that are power consumers
(and their respective power consumption),
and the net power recovered by
the system.
Power-recovery rates will vary for
different applications based on fuel
CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2010 37
D-100
Fluegas
outlet
185°F
5
T-100
E-101
http://WWW.CHE.COM
Chemical Engineering September 2010

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