Chemical Engineering January 2013 - 35

100
110
120
Cooling
water inlet
Cooling
water outlet
Condenser
60
70
80
90
ISO
design
GT power at 35°C
5 10 15 20 25 30 35 40 45 50
Air compressor inlet air temperature, °C
FIGURE 10. The effect of inlet air temperature on the power
output of a typical gas turbine is shown [22]
the fuel, amount of excess combustion
air used and humidity of air.
An indirect-contact condensing type
of economizer can heat deaerator feedwater
to a higher temperature compared
to the direct-contact condensing
type. The possibility of corrosion from
the acidic condensate is prevented by
using more expensive materials like
stainless steel and glass fiber for ducts
and stacks, or by coating exposed
metal surfaces with a resistant material,
such as Teflon.
Condensation systems generally reduce
particulate and SOx emissions.
The penalty for firing with excess air
decreases with reduction in fluegas
temperature. Condensing economizers
are difficult to implement (due to corrosion
issues), if the boiler is sharing a
flue stack with other fired equipment.
Condensing economizers are applicable
only if there is a requirement for
hot water.
Economizers for fired heaters. If
fluegas temperatures are very high
(~700°C), then adding a new coil to
the convection section can increase
furnace capacity and reduce fuel consumption
by bringing the fluegas temperature
to within 50-100°C of the
process-fluid inlet temperature at the
inlet of the convection section [20].
Combustion air heating for boilers
and fired heaters. An air pre-heater
(APH) is a heat exchanger placed in
the exhaust stack or ductwork that
extracts a large portion of thermal energy
in the fluegases and transfers it
to the incoming combustion air. WHR
from stack gases through air preheating
proves to be more advantageous
than other methods [15]. This practice
also reduces required capacities
of forced- and induced-draft fans because
the combustion air quantity is
reduced. The amount of energy saved
Economizer
Evaporator
Waste heat source inlet
Waste heat
source outlet
FIGURE 11. This diagram depict an organic Rankine cycle
(ORC) with an economizer (internal heat exchanger)
through APH can be very large since
stack temperatures can be reduced to
below 180°C (depending on the fluegas
acid dew point). APHs require space,
but energy savings can be as much as
20-30%, for the case of fired heaters.
There are two types of air preheaters:
recuperator and regenerator.
A recuperator is a fixed air-to-fluegas
heat exchanger (without moving parts)
placed in the furnace stack to preheat
incoming air with hot fluegas. A regenerator
is an insulated container filled
with metal or ceramic shapes that
can absorb and store relatively large
amounts of thermal energy and then
release that energy subsequently. Another
design of a regenerator for continuous
operation uses a continuously
rotating wheel containing a metal or
ceramic matrix. The fluegases and
inlet stream (such as combustion air)
pass through different parts of the
wheel during its rotation to receive
heat from the fluegases and release
heat to the cold, inlet stream.
Installing APHs for smaller heaters
(with absorbed duties of 7,000 to
10,000 kW and less) may not meet
payback period requirements for most
petroleum refineries. The payback period
for installation of an economizer
or APH depends mainly on the fluegas
flowrate, stack temperature, annual
operating hours and the need for hot
water. An economizer or APH for water-tube
boilers is typically not attractive
for units operating under 10 kg/
cm2g or below 20 ton/h of steam production,
nor any size boiler that will
normally run at reduced capacity. For
industrial boilers, dual installation
using both an economizer and an APH
is rarely economical or installed.
Heat recovery using a heat-pipe. A
heat pipe is a heat transfer element
that can quickly transfer heat from
one point to another with merits of
high efficiency and compact size. Its
heat transfer coefficient in the evaporator
and condenser zones is 1,000-
100,000 W/m2K, and its thermal resistance
is 0.01-0.03 K/W, thus leading
to a smaller area and mass of a given
heat exchanger [42]. The mechanism
of heat pipes is to employ evaporative
heat transport to transfer thermal
energy from one point to another
by evaporation and condensation of a
working fluid or coolant.
Because a heat pipe cannot function
below the freezing point nor above
the critical temperature of its working
fluid, the selected working fluid
must be within this range. In addition,
vapor pressure, surface tension,
contact angle and viscosity in the heat
pipe must be considered in the selection
of a working fluid [26]. Working
fluids that can be used in low-temperature
heat recovery include methanol
(10-130°C), flutec PP2 (10-160°C),
ethanol (0-130°C), water (30-200°C)
and toluene (50-200°C). Heat-pipes
can be used as APHs in steam boilers,
but their installations are limited,
largely due to higher costs. Another
major limitation of heat pipes is that
they must be tuned to particular cooling
conditions. The choice of pipe material,
size and coolant all have an
effect on the optimal temperatures in
which heat-pipes work.
Gas turbine (GT) inlet air heater.
GT inlet air (after air compression)
can be heated with fluegases. Also,
BFW can be preheated by installing
an economizer in a heat recovery
steam generator).
Heat reuse in other equipment
Fired heaters. Fluegas heat can be
used for steam generation in the economizer,
steam superheater and hot oil
CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2013 35
Electrical
generator
Turbine
% of GT's rated capacity
http://WWW.CHE.COM
Chemical Engineering January 2013

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