Note: Descriptions are shown in the official language in which they were submitted.
CA 02473033 2004-06-30
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CA 02473033 2004-06-30
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TITLE OF THE INVENTION:
A method of increasing the efficiency of a fuel gas turbine by cooling
combustion air
FIELD OF THE INVENTION
The present invention relates to method of increasing the efficiency of a fuel
gas
turbine by cooling combustion air.
BACKGROUND OF THE INVENTION
An increase in combustion air temperature to a fuel gas turbine results in a
decrease in mechanical output. This is because, at different ambient air
temperatures, the
mass flow rate changes due to a change in air density. Since combustion
processes are
based on mass, this change in air mass flow rate changes the combustion
conditions. As a
result, the available shaft power at higher ambient temperatures decreases,
resulting in a
decrease in performance.
SUMMARY OF THE INVENTION
What is required is a method of increasing the efficiency of a fuel gas
turbine by
cooling combustion air.
2 0 According to the present invention there is provided a method of
increasing the
efficiency of a fuel gas turbine by cooling combustion air. A first step
involves providing a
heat exchanger having a first fluid path and a second fluid path, such that a
heat exchange
takes place between fluids passing along the first fluid path and fluids
passing along the
second fluid path. A second step involves lowering fuel gas pressure to match
fuel inlet
2 5 pressure requirements for the fuel gas turbine, with the lowering in
pressure resulting in a
lowering in temperature of the fuel gas. A third step involve s directing low
temperature fuel
gas through the first fluid path of the heat exchanger and combustion air
through the second
fluid path of the heat exchanger, such that a heat exchange takes place that
raises the
temperature of the fuel gas and lowers the temperature of the combustion air.
With the method, as described above, combustion air is cooled at relatively
little cost
through a heat exchange with the fuel.
CA 02473033 2004-06-30
2
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will become more apparent from the
following description in which reference is made to the appended drawings, the
drawings are
for the purpose of illustration only and are not intended to in any way limit
the scope of the
invention to the particular embodiment or embodiments shown, wherein:
THE FIGURE is a flow diagram of a supply of combustion air to a fuel gas
turbine
in accordance with the teachings of the present method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred method of increasing the efficiency of a fuel gas turbine by
cooling
combustion air will now be described with reference to THE FIGURE.
The invention relates to the cooling of combustion a:ir to fuel gas turbines.
This
process increases the amount of the work available at the gas turbine shaft.
The increment
in efficiency on these gas turbines drivers be it for a power generator or a
mechanical load
is dependent on ambient temperature.
At present, the available shaft output on a fuel gas turbine is identified by
a curve
correlating ambient air temperatures and mechanical output. Typically, this
curve shows
that an increase in ambient inlet air temperature results in a decrease in
mechanical output.
This is because the volumetric flow rate is constant while at different
ambient air
2 5 temperatures the mass flow rate changes due to a change in air density.
Now, since
combustion processes are based ~n mass, this change in air mass flow rate
changes the
combustion conditions. As a result, the available shaft power at higher
ambient
temperatures decreases, resulting in a decrease in performance.
3 0 The invention proposes to utilize the cold energy available in the fuel
gas that is
consumed in the fuel gas turbine to pre-cool the ambient air to the desired
temperature as
per THE FIGURE.
CA 02473033 2004-06-30
3
It is proposed that the fuel gas from the gas grid 12 first be expanded in
expander
14 to the desired Iower temperature. Whenever there is a pressure reduction in
the gas, the
temperature of the gas will drop. This is referred to as the Joules-Thompson
effect.
Depending upon the pressure drop and the flow the temperature can drop to or
below the
freezing point. This low temperature stream 16 is then recovered through an
exchanger 18
by a fluid in line 20 which is then used to pre-cool the combustion air in air
heat
exchanger 22 to the fuel gas turbine 24. The fluid in line 1R is in a closed
circuit system
whereby its only purpose is to recover the coldness from the fuel gas from gas
grid 12 and
exchange it to the combustion air supply to the fuel gas turbine 24.
The fuel gas is then recompressed (where applicable) to its prerequisite
pressure
for combustion. The end result is a net gain on available power. Furthermore,
by
maintaining the fuel gas turbine 24 operating at constant conditions it is
expected that its
maintenance requirements will decrease due to less temperature fluctuations on
the turbine
24.
Power is generated by gas turbines, the gas is typica~'.ly supplied from a
natural gas
pipeline distribution network. A gas turbine power generation rating is based
on ISO
conditions (15 °C, sea level). This is due to the impact on power
generation by inlet air
2 0 density changes. These changes are due to ambient air temperature and
pressure.
Air enters the gas turbine for both combustion and cooling purposes. The air
is
compressed by an axial compressor, a volumetric machine which always draws the
same
volume of air. If the air density is lower , the mass drawn in will be lower ,
thus the
2 5 combustion will be less efficient. If the air density is higher ( lower
ambient temperature )
combustion occurs at higher temperatures, the flue gases expand more
efficiently and,
more power is generated. In fact, all gas turbines have a performance curve
directly
relating available power with ambient temperature. Therefore, cooling down the
air
entering the gas turbine, generates more power .
The cool flow concept achieves this with a simple and economic process. As
shown in THE FIGURE, the fuel gas enters the system from the gas grid 12 and
enters
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CA 02473033 2004-06-30
the expander 14 where it expands to the gas pressure required to operate the
turbine 24.
Typically the gas grid pressure is about 50 to 70 barer and the temperature
around 20°C,
whereas the gas entering the gas turbine is typically 20-24 barC .
The fuel gas from the gas grid 12 enters the expander 14 where the pressure
drops
to match the inlet pressure requirement of the fuel gas turbine 24, as a
result the
temperature also drops due to the Joules-Thompson effect.
The cool gas enters a gas heat exchanger 18 where it gives up its cold energy
to a
cooling fluid in a closed circuit 20. The cooling fluid is circulated under
pressure, to
absorb the cold energy from the fuel gas in the heat exchanger 18, by
conduction.
The fuel gas having given up its cold energy, exits the gas heat exchanger 18
through line 28 and enters the gas turbine 24 as fuel. The pressurized and now
colder
cooling fluid in closed circuit 20 exits the gas heat exchanger 18 and enters
an air heat
exchanger 22.
The inlet air 30 to the axial compressor 34 on the fuel gas turbine 24 is
first cooled
on the air heat exchanger 22 by the cold cooling fluid stream in closed
circuit 20. The
2 0 now colder air 32 exiting the air heat exchanger 22 has a higher density
thereby increasing
the air mass flow rate to the axial compressor 34. Power is output from the
turbine 24
from output 38. The increment in mass flow rate dictates the increment in the
fuel gas
turbine performance. This performance can be predicted from the performance
curves
supplied by the fuel gas turbine manufacturer.
The cooling fluid in closed circuit 20 exits the air heat exchanger 22 and
enters the
cooling fluid storage vessel 36. The cooling fluid is then circulated by a
variable speed
pump 26. The cooling fluid flow rate is controlled by a temperature controller
36 on the
air stream 32.
The cooling system is a closed system whereby the objective is to capture and
release the cold energy.
CA 02473033 2004-06-30
Gas-Coolant exchange equation
Qc = Mc Cp ( Tin - Tout )
5 Where Qc = heat needed to be exchanged
Mc= mass of the coolant
Cp = Specific heat of the coolant
Tin = inlet coolant's temperature
Tout = outlet coolant's temperature
To calculate the amount of heat you need to exchange the gas with the air
entering
the gas turbine
Q = M Cp ( Tout - Tin )
Where Q air = heat you need to exchange
M air = mass of the air , date supplied by the turbine manufacturer.
Cp air = Specific heat for air
2 0 Tin = air's inlet 's temperature
Tout = air's outlet temperature
The body of the invention can also be utilized at the intercooler stage of any
compressor. Again the objective being to increase the mass per unit volume of
the
2 5 compressible fluid.
The following graph shown in Table 1 is a graph of a typical Gas Turbine
Generator normally supplied the manufacturer, it shows how a difference of
30°C (from 5
to 35°C) in ambient air temperature results in a difference in
performance of 2500 Kw or
3 0 21.5% in power production.
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CA 02473033 2004-06-30
In this patent document, the word "comprising" is used in its non-limiting
sense to
mean that items following the word are included, but items not specifically
mentioned are not
excluded. A reference to an element by the indefinite article "a" does not
exclude the
possibility that more than one of the element is present, unless the context
clearly requires that
there be one and only one of the elements.
It will be apparent to one skilled in the art that modifications may be made
to the
illustrated embodiment without departing from the spirit and scope of the
invention as
hereinafter defined in the Claims.
Table 1 - Power Generarion vs. Amnieni Hir a CmpG~aw~
