U.S. patent application number 10/092487 was filed with the patent office on 2003-01-09 for natural gas depressurization system with efficient power enhancement and integrated fail safe safety device.
Invention is credited to Walpita, Nalin.
Application Number | 20030005699 10/092487 |
Document ID | / |
Family ID | 26785736 |
Filed Date | 2003-01-09 |
United States Patent
Application |
20030005699 |
Kind Code |
A1 |
Walpita, Nalin |
January 9, 2003 |
Natural gas depressurization system with efficient power
enhancement and integrated fail safe safety device
Abstract
The instant invention provides a gas depressurization station,
comprising a preheater for preheating a gas, an expander having an
output shaft for expanding and depressurizing the gas, and an air
compressor coupled to the output shaft for creating high pressure
air for use in controlling the depressurization process. The
invention is also directed to a method for safely depressurizing a
gas, comprising: passing the gas through a preheater and a gas
expander which drives an output shaft; driving an air compressor
attached to said output shaft to produce compressed air; and
controlling the pressure of said compressed air by means of control
throttling means; whereby unwanted variations in the speed of
rotation of said output shaft may be safely controlled by said
control throttling means.
Inventors: |
Walpita, Nalin; (Colombo,
LK) |
Correspondence
Address: |
Eugene E. Renz, Jr. P.C.
205 North Monroe Street
Media
PA
19063-9056
US
|
Family ID: |
26785736 |
Appl. No.: |
10/092487 |
Filed: |
March 8, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60275094 |
Mar 12, 2001 |
|
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Current U.S.
Class: |
60/653 |
Current CPC
Class: |
F02C 3/36 20130101; F02C
1/00 20130101; Y02E 20/16 20130101 |
Class at
Publication: |
60/653 |
International
Class: |
F01K 007/34 |
Claims
What is claimed is:
1. A gas depressurization station, comprising a preheater for
preheating a gas, an expander having an output shaft for expanding
and depressurizing the gas, and an air compressor coupled to the
output shaft for creating high pressure air for use in controlling
the depressurization process.
2. The gas depressurization station of claim 1, wherein some of the
air output from said air compressor is used to preheat the gas in
said preheater.
3. The gas depressurization station of claim 1, wherein some of the
air output from said air compressor is used to generate power.
4. A gas depressurization station, comprising a preheater for
preheating a gas, an expander having an output shaft for expanding
and depressurizing the gas, and a fail-safe safety system
comprising: an air compressor coupled to the expander output shaft;
and control throttle means to throttle the air output from said air
compressor; whereby unwanted variations in the rotation speed of
said shaft may be controlled by said control throttle means.
5. The gas depressurization station of claim 4, wherein said
control throttle means comprises a pressure reduction valve and a
fail safe main air pathway shutoff valve connected in parallel to
the output of said air compressor.
6. The gas depressurization station of claim 4, wherein some of the
air output from said air compressor is used to preheat the gas in
said preheater.
7. The gas depressurization station of claim 4, wherein some of the
air output from said air compressor is used to generate power.
8. A method for safely depressurizing a gas, comprising: passing
the gas through a preheater and a gas expander which drives an
output shaft; driving an air compressor attached to said output
shaft to produce compressed air; and controlling the pressure of
said compressed air by means of control throttling means; whereby
unwanted variations in the speed of rotation of said output shaft
may be safely controlled by said control throttling means.
9. The method of claim 8 for safely depressurizing a gas, wherein
some of the compressed air produced by said air compressor is used
to preheat the gas in said preheater.
10. The method of claim 8 for safely depressurizing a gas, wherein
some of the compressed air produced by said air compressor used to
generate power.
Description
[0001] This application claims priority from U.S. provisional
application No. 60/275,094 filed Mar. 12, 2001.
FIELD OF THE INVENTION
[0002] The invention is applicable in the area of natural gas
depressurization wherein there is enhancement to the power that may
be generated by preheating the gas. This enables each
depressurization gate station to produce a substantial quantum of
power, with a heat rate which is 50-35% better then the existing
best gas turbine power plant.
DESCRIPTION OF THE PRIOR ART
[0003] Depressurization of natural gas is conventionally done by
means of a throttling valve. The depressurization action is an
isenthalpic process, known as a Joule-Thompson expansion. Because
of relationships between gas properties and inlet and outlet
pressures, the J-T expansion can result in either a temperature
increase or decrease. The controlling parameter is known as the
Joule Thompson temperature inversion curve and it can be plotted
for any gas.
[0004] If energy is extracted from the gas, then the process would
be known as a reversible, adiabatic expansion, if the expanding gas
is kept in complete thermal isolation. Due to a reduction in
enthalpy of the gas, there is substantial cooling taking place. An
earlier provisional patent application looks at the possibility to
acquire heat from the atmosphere, by expanding in three stages and
communicating with the atmosphere in between stages.
[0005] A further important aspect of natural gas depressurization
with power generation is that safety of the system must be
maintained at all times. In the case of a throttling valve, the
element (piston) of the valve which provides the orifice through
which the pressure step down takes place, moves up and down in
response to pressure signals in the downstream distribution circuit
as such. All changes are accommodated.
[0006] The element of risk arises especially if the valve or in the
case of depressurization with power generation, the expander device
allows excessive flow through onto the downstream side, without any
demand for such flow. This can happen if the element in the
throttling valve sees restricted movement or if there is a grid
trip, in the case of an expander coupled to a grid feeding
alternator.
[0007] In the latter case above, where power is being fed to the
grid, this ensures that expander and alternator run at grid
frequency and all changes in mechanical power at the expander are
converted to changes in electrical power, without any change of
speed. This is a property of synchronous alternators connected to a
grid which is much larger than the generator output. However, if
the grid is tripped or disconnected from the alternator, this
condition no longer applies. The mechanical power flow then has
nowhere to go and the result is an immediate speeding up of the
expander and alternator. In the case of a volume based expander,
speeding up will mean more volumes being passed through per second,
resulting in possible overpressurization of the downstream circuit,
with no regard to actual downstream demand.
[0008] Since downstream structures and end users are designed for
gas pressures within a narrow range, the overpressurization will
result in a substantial safety hazard. Whilst an electrical system
to provide a dummy load would be switched in every time the grid
trips, enabling the speed to remain at approximately the value at
the time of the trip, there is concern that an electrical failure
in this dummy load system could still put the system at risk.
Further, the simple mechanical failsafe system which is part of the
throttle valve is not matched in terms of durability and
reliability, in an electrical dummy load activation system.
[0009] Williams U.S. Pat. No. 6,155,051 looks at running a
compressor with a natural gas expander, to provide the necessary
heat to preheat the gas. The following points are noteworthy,
concerning this patent: (1) The '051 patent uses only a portion of
the gas to provide power by gas expansion; (2) The '051 patent only
purpose in compressing air is to provide heat of expansion to the
incoming natural gas; and (3) another purpose in preheating the
natural gas is to reduce Joule-Thompson cooling which may occur in
the throttling valve process. No further use is made of the
compressed air.
[0010] The present invention provides a mechanical failsafe system
equal in simplicity and durability to the failsafe operating mode
of the throttle valve, for use together with and to back up the
dummy load type failsafe system.
[0011] In natural gas depressurization, adding a quantum of preheat
to the gas prior to entry to the expander will enable the gas to
expand without any cooling of the gas below atmospheric
temperature. Typically, for gas incoming at 1000 psig and outgoing
at 150 psig, increasing the incoming temperature to 115.degree.
Celsius will result in a temperature after expander of around
0.degree. C. However, a portion of the natural gas will have to be
consumed for this purpose.
[0012] Because the gas is already compressed at entry to expander,
the heat rate for the process is up to 50% less than for a typical
Gas Turbine Combined Cycle power plant with an overall thermal
efficiency of 55%. In all such conventional plants, compression
power is fed back from the expander output, which is not required
in this case.
[0013] The present invention is directed to a system to maximize
the power output from each such depressurization location, without
loosing the benefit of a good heat rate. Further, direct heating of
natural gas to a temperature of 115.degree. C. with a gas flame at
a temperature of 1,750.degree. C. will result in a substantial loss
of available energy or exergy. The invention therefore embodies a
method whereby the exergy loss could be minimized. Further, the
invention embodies a system which uses a property of one of the
mechanical devices to provide a failsafe, mechanical means of
preventing overpressure protection. Further, the invention
incorporates a mathematical formulation concerning establishing the
optimum plant sizing for the enhancement of power output.
BACKGROUND TO THE INVENTION
[0014] Natural gas is compressed to high pressure for transmission
from wells to consumers, typically the pressures in the main
interstate pipelines are of the order of 1000-3000 psig. On the
other hand, natural gas is distributed to consumers at much lower
pressures. Typically, for domestic consumers, the pressure may
20-40 psig and for commercial consumers, 100-150 psig.
[0015] The pressure in the interstate high pressure transmission
pipeline is reduced to the distribution pressure in a "gate
station", by means of a pressure reducing throttling valve. Several
types of throttling valve are to be found, the main types being
Direct Operated Valves and Pilot Operated Valves.
[0016] The change of pressure between the high pressure
transmission pipe and the low pressure distribution side
constitutes a loss of potential energy. If the gas is placed within
a suitably conFigured expansion system such energy may be usefully
utilized to produce shaft work, which may be converted to electric
power.
[0017] However, there are crucial differences between the expansion
and pressure drop in a throttling valve and that in a power
generating device. Most crucially, the former is theoretically an
isenthalpic or constant enthalpy expansion, typically known as a
Joule-Thompson expansion. In such an expansion, depending on the
conditions, the outgoing temperature may be lower or higher than
the incoming temperature. The governing parameter is known as the
Joule Thompson inversion temperature curve In particular instances
where there is significant temperature drop, heating is required to
bring the gas temperature back to a reasonable level
[0018] In the case of a direct gas expansion with power generation,
where the gas expands in isolation, the theoretical description is
isentropic or adiabatic, reversible expansion. External work or
energy is extracted from the gas stream, this means the outlet
enthalpy of the gas is lower than the inlet enthalpy. Invariably,
this means the outlet temperature is lower than at inlet. Because
of the substantial pressure difference between the incoming main
and distribution main, if a direct expansion is carried out, the
result is a very low gas temperature exiting the expander, see FIG.
1.
[0019] Such low temperatures are unacceptable because a substantial
number of the components in the gas will liquefy or solidify.
Therefore a method is required to capture the useful energy in the
pressure difference between transmission mains and distribution
mains, without substantial temperature decrease. A simple method
suggested has been to preheat the incoming gas by burning a portion
of the natural gas.
[0020] Preheating of the gas to around 115.degree. C. will enable
the following conditions to be achieved:
[0021] Typical Case
1 Natural gas flow 10 Kgs/sec Power generation 2340 kW Heat input
1900 kW Heat rate 2950 kJ/kWh Final gas temperature 0.degree.
C.
[0022] The average heat rate in the case of a Gas Turbine Combined
Cycle with a thermal efficiency of 55% overall would be
approximately 6545 BTU/kWh. Because the compression power need not
be subtracted in the present case, the heat rate is only 50% of the
GTCC case quoted, indicating a low fuel consumption compared with
other cycles where compression power is fed back from the
output.
[0023] If most of the gas is diverted through an expander, coupled
to an alternator, then if there is a trip in the grid, the expander
would overspeed, resulting in overpressurization of the downstream
circuit During grid connected operation, because the grid is large
compared with each individual alternator, the frequency and hence
the speed of rotation remains constant. This condition does not
apply when the alternator is disconnected from the grid. The back
EMF and hence the reverse torque velocity vector which opposes the
input torque to the alternator (from the expander) collapses. The
input torque then has a tendency to accelerate the rotating masses
until one of several mechanisms is able to absorb the incoming
power.
[0024] In the case of the expander, under conditions where the
alternator is disconnected, the pressure difference causes the
machine to speed up until (a) the pressure difference is lessened
due to over pressurization of the downstream circuit and (b) the
mechanical power is dissipated through fluid friction within the
expander.
[0025] Under these conditions the expander reaches an equilibrium
speed, typically in the case of Francis type water turbines in
hydro applications, this is 2.2 times the synchronous speed. In
hydro applications, discharging more water after the grid is
disconnected does not matter. This is certainly not the case in
natural gas depressurization. Any tendency to overspeed, followed
by excessive pressurization of the downstream circuit, is
completely unacceptable.
[0026] The present invention therefore embodies unique means to
mechanically control and absorb the output on grid disconnection,
without imposing any load during grid connected operation, or by
imposing a load which leads to further benefit.
[0027] The benefit realized by imposition of a mechanical device is
enhanced, in this invention, by further use of the output from the
mechanical safety device. The invention therefore embodies the
integration of both a safety device and a power generation device,
for substantial added benefit.
SUMMARY OF THE INVENTION
[0028] The present invention is directed to a gas depressurization
station, comprising a preheater for preheating a gas, an expander
having an output shaft for expanding and depressurizing the gas,
and an air compressor coupled to the output shaft for creating high
pressure air for use in controlling the depressurization
process.
[0029] The instant invention also provides a gas depressurization
station, comprising a preheater for preheating a gas, an expander
having an output shaft for expanding and depressurizing the gas,
and a fail-safe safety system comprising: an air compressor coupled
to the expander output shaft; and control throttle means to
throttle the air output from said air compressor; whereby unwanted
variations in the rotation speed of said shaft may be controlled by
said control throttle means.
[0030] Furthermore, the invention provides a method for safely
depressurizing a gas, comprising: passing the gas through a
pre-heater and a gas expander which drives an output shaft; driving
an air compressor attached to said output shaft to produce
compressed air; and controlling the pressure of said compressed air
by means of control throttling means; whereby unwanted variations
in the speed of rotation of said output shaft may be safely
controlled by said control throttling means.
BRIEF DESCRIPTION OF THE FIGURES
[0031] FIG. 1 describes graphically the relationship between final
temperature and final pressure in direct expansion of the gas.
[0032] FIG. 2 illustrates a brief schematic of the invention.
[0033] FIG. 3 shows the safety system schematic of the
invention.
[0034] FIG. 4 describes an alternative embodiment of the safety
system.
[0035] FIG. 5 illustrates the thermodynamic cycle diagram of the
invention.
[0036] FIG. 6 describes in detail the schematic of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The present invention provides the following novel
developments in the technology of gas transmission and safety
handling:
[0038] A system to depressurize natural gas wherein the power
generated by a natural gas expander is used to compress air, as the
first step in a combined cycle.
[0039] Air compressed through natural gas expansion is preheated
and expanded in an air expander, to generate power.
[0040] The combination of a first natural gas expander, followed by
an air compressor, followed by an air heater, followed by a hot air
expander, wherein extra nett output is produced over and above the
power available in natural gas expansion only.
[0041] A system where heat is provided by natural gas combustion to
achieve a high temperature in an air stream thereby utilizing a
substantial quantum of the availability in a high temperature
flame.
[0042] A system which recovers heat after expansion in an air
expander and preheats the air stream from the compressor in an
exhaust gas recuperator, prior to entry to a natural gas fired
heater.
[0043] A system which first provides heat of compression of air to
the natural gas stream, then acquires heat from the recuperation of
hot exhaust air from an air expander, then is heated to a high
temperature by natural gas combustion and is then expanded to
produce useful work.
[0044] A system in which the flow and pressure of air from an air
compressor driven by a natural gas expander may be independently
controlled, provided the total energy change (increase)in the air
stream is equal to the energy change (decrease)from the natural gas
expansion process.
[0045] A system in which there is a critical value of air flow from
an air compressor driven by a natural gas expander wherein the heat
required for preheating natural gas exactly equals the sensible
heat available in the compressed air flow, resulting in a system
where in the ideal case there is no heat rejected from the total
system
[0046] A system in which for air flow above the critical value,
there is heat rejected to atmosphere and for air flow below the
critical value, combustion of natural gas directly for preheating
the natural gas, is required.
[0047] A power generation system in which the heat rate is 35-50%
better than a conventional gas turbine combined cycle because of
the availability of a compressed fluid medium, which may
expanded.
[0048] A power generation system which recovers hitherto wasted
energy whilst further optimising the potential at each gate station
site.
[0049] A power generation system consisting of a first natural gas
expander to which preheated natural gas is supplied, followed by a
compressor which is mechanically driven by the first expander,
followed by an alternator to which the compressor is mechanically
connected by a shaft, followed by a second air expander to which
the compressor is mechanically connected by a shaft, in parallel
the air compressed is used to preheat the natural gas, it is then
heated in an exhaust gas recuperator, then further heated to a high
temperature in a natural gas heater, then expanded in an air
expander.
[0050] A mechanical, shaft power circuit consisting of a first
natural gas expander, followed by a second, air compressor,
followed by a third, alternator or synchronous generator, followed
by a fourth unit, which is actually a second expander, all on a
common shaft.
[0051] An air circuit consisting of a first compressor, followed by
a heat exchanger to provide heat to natural gas, followed by a
recuperator to heat incoming air, followed by a gas firing heat
exchanger to further increase output, followed by an air expander
wherein the enthalpy change in the air is converted to shaft
power.
[0052] A natural gas circuit consisting of a heat exchanger to
carry out air preheat, followed by a natural gas expander where the
change in enthalpy is converted to shaft power.
[0053] An integration of the mechanical power, air and natural gas
circuits to provide a system of safe natural gas depressurization
with generation of additional shaft power and with zero heat
rejection to atmosphere, under certain conditions, resulting in
high overall efficiency and low heat rate.
[0054] A safety system in which the air compressor itself is used
to control the speed of the natural gas expander and hence the air
thoughflow, by virtue of using a throttled flow a compressed air to
absorb the power generated in the natural gas expander.
[0055] A safety system in which a simple gravity driven system is
used to shutoff air flow to the heating and recuperation
system.
[0056] A system in which a spring operated pressure reducing valve
of the very same nature as the conventional throttling valve is
used to throttle air flow from the compressor, thereby absorbing
power generated by the natural gas expander, in a controlled
manner.
[0057] A system in which the rotating natural gas expander and air
compressor closely coupled to it act as a mechanical, fail safe,
safety device, to eliminate any possibility to over-pressurization
in the downstream circuit, by means of. a throttle valve which
controls the air stream from the compressor.
[0058] A safety system in which on grid failure, the solenoid
holding off valve colsure is denergised, allowing main flow circuit
valve to close, thereby pressurising the air compressor, which
causes the parallel throttle valve circuit to open, followed by
control of the air flow by this throttle valve, which leads to
effective control of the rotational speed of the compressor and
natural gas expander
[0059] The invention consists of a natural gas preheater, a natural
gas expander, an air compressor and air expander, as shown in FIG.
2. The air compressor also acts as the mechanical failsafe safety
element which is an essential requirement for safe operation.
[0060] The natural gas expander and air compressor are integrally
connected through a shaft coupling. The fail safe safety system as
embodied in the invention involves shutting down the main air flow
valve within 10 milliseconds of grid failure and allowing the
compressor safety valve to open, see FIG. 3.
[0061] The safety system consists of a pressure reduction valve
connected in a parallel path to the compressor high pressure side,
and a fail safe main air pathway shutoff valve. On grid failure,
the air pathway to air heater and expander is closed off and all of
the compressor air flow is diverted to the pressure relief valve
indicated. The pressure relief valve then acts in an exactly
analogous manner to the original gate station PRV and would, in
fact be the same type of valve. It controls the pressure
development within the compressor, thereby controlling compressor
power input and hence power absorbed from NG expander.
[0062] This present invention also embodies means by which gas
pressure may be controlled in a simple way, in an analogous manner
to the original gate station pressure control system and employing
a standard pressure reduction valve, The gas flow volume passing
through the expander is now under complete control by these means.
Only air passes through the compressor and it's PRV circuit.
[0063] The invention embodies means whereby natural gas pressure
can be controlled as the gas is passed through an expander, by
employing an air compressor directly coupled to the expander shaft
and throttling the air output from the air compressor.
[0064] Furthermore, the invention embodies means to usefully
utilize the excess power available in always rotating the
compressor, by using compressor output in a thermodynamic
cycle.
[0065] Referring to FIG. 2, the natural gas first passes through a
preheater, then through a natural gas expander which enables the
enthalpy change to be converted to shaft power. The natural gas is
then discharged directly to the distribution network at the correct
lower pressure. The expander is mechanically connected to the air
compressor, which also acts as the primary element in the safety
device, see FIG. 3. In normal operation where the unit is connected
to the grid, the compressed air proceeds to the natural gas air
preheater, where it's temperature drops substantially. Thereafter,
it proceeds to the recuperator and then main air heater, whereby
through combustion of natural gas, the temperature is raised to
between 900-1100.degree. C. The heated air is then expanded in the
air expander and the resulting shaft work is supplied to the
alternator.
[0066] The alternator receives shaft work from both the NG expander
and compressor and from the air expander, through shaft projections
at either end. The shaft work supplied to the alternator from the
NG expander is nett of the work supplied to the compressor. After
air expansion, the air still has sensible heat remaining within it,
see cycle diagram, FIG. 5. This sensible heat is given up to the
incoming cold compressed air in the recuperator.
[0067] The invention also embodies means whereby in the case of the
air cycle, there is no heat rejected to atmosphere, see cycle
diagram, FIG. 5. The closed air cycle, as such, does reject heat,
from the compression step to the natural gas, therefore there is no
contravention of the Second Law of Thermodynamics. In the
particular circumstances pertaining, the natural gas heating and
expansion is an open cycle process, hence it is not governed by the
Second Law of Thermodynamics. The incoming preheat is converted
into shaft work and the gas input and output enthalpy remain the
same, with slight variation due to the change in pressure, in the
case of a real gas.
[0068] The invention therefore embodies means in certain
configurations whereby with the integration of a closed cycle air
cycle and an open cycle natural gas expansion process, no heat is
rejected to the atmosphere. In cases where the airflow is large in
comparison with the natural gas flow, there is more heat in the
compression process than required for natural gas expansion and in
such cases a quantum of heat would be rejected to atmosphere. In
other cases, correspondingly, where the air flow is small in
comparison with natural gas flow, there is insufficient heat in the
compressed air--in such cases additional heat has to be provided,
by burning natural gas.
[0069] The invention further embodies means to enhance the power
output from a natural gas depressurization station, by connecting
the natural gas expansion process to a closed Brayton type air
cycle, with recuperation. The sensible heat in the air after
compression will be transferred to the natural gas thereby enabling
very efficient recuperation. In the typical gas turbine case,
recuperation can only take place between the outgoing air
temperature and the compressed air temperature, hence the heat of
compression is effectively lost, unlike in this case.
[0070] With reference to a usual Brayton cycle, which can be
represented as an adaibatic compression, followed by a constant
pressure heat addtion, followed by an adiabatic expansion, followed
by a constant pressure heat rejection, 100% recuperation is never
possible because of the elevated temperature of the gas or air,
after compressor
[0071] The invention is also directed to a natural gas
depressurization system and a fail safe safety system in an
integrated device, with substantial synergies arising from the dual
use of one and the same item., namely, the air compressor
[0072] The natural gas expansion system consists of a first
preheater, HEATEX I in the main flow diagram, FIG. 6, overleaf
Natural gas enters at I at ambient condition and main pipeline
pressure, shown here as 1000 PSIG. It is then preheated by the
sensible heat in compressed air entering at 9 and leaving at 10.
The natural gas acquires a temperature well in excess of the
ambient typically between 100-250.degree. Celsius, a temperature of
115.degree. C. is indicated in the example.
[0073] The natural gas at slightly less than pipeline pressure and
with an elevated temperature is introduced to the first expander,
EXP 1, where the gas is depressurized from 1000 psig to 150 psig in
the example, the incoming and outgoing pressures could be at
various levels in other examples and cases. In the course of the
depressurization, the enthalpy change is converted into shaft
power. At 5, the expander is connected up with a standard air
compressor. The compressed air flow may be determined independently
of the natural gas flow and constitutes a first independent
variable. The temperature and pressure of air exiting compressor is
192.degree. C. and 74 psig respectively, in this example. In the
example shown the flow of air is I 1 Kg/second. The compression
pressure may also be set quite independently of the natural gas
system pressure variations and constitutes a second independent
variable. In the example, the pressure ratio is 5:1, giving an
outlet temperature and pressure from the compressor of 192.degree.
C. and 75 psig. Many other combinations of pressure, temperature
and flow may be adopted.
[0074] After the expansion of natural gas to the requisite lower
pressure has taken place, a small portion of the gas flow,
typically 2-5%, is diverted for heating purposes. This is shown as
an offiake line at point 4 in FIG. 6. The hot air from the
compression process is now diverted to the first heat exchanger,
HEATEX1, where after transferring sensible heat to the natural gas
flow, the colder compressed air is sent out at 10 and at point 11,
enters a recuperator, RECUP 1, which is also a heat exchanger.
RECUP 1 has the task of transferring sensible heat from the exhaust
air flow leaving the air expander, to the incoming cold compressed
gas In RECUP1, the temperature of the incoming compressed air is
increased to an approach value of around 10-30.degree. C.
[0075] At this point in the description, the thermodynamic
pressure-volume diagram given in FIG. 5 will further illustrate the
concept. The natural gas entry, preheating and expansion are
represented by the thick line A-B-C. The heavy dotted line is the
envelope demarcating the ambient temperature. The natural gas heat
pick up in process A-B is exactly equal to the compressed air heat
release in the section E-F in the air cycle diagram.
[0076] After the compressed air has transferred sensible heat to
the natural gas, it is reheated in the recuperator, the relevant
process line is F-E' in FIG. 5. The heat pickup in section F-E' is
equal to or less than the heat given up in section H-C, which
represents the process on the other side of the recuperator, je
where the exhaust air transfers heat and is cooled to near
atmospheric temperature. It is apparent that in a theoretical or
ideal diagram, since heat rejected by the air cycle, represented by
H-C, is fully absorbed by the natural gas, after which a conversion
to shaft power takes place, there appears to be no reject heat,
leading to a contravention of the Second Law of Thermodynamics. In
fact, the Second Law cannot be applied overall because the natural
gas forms part of an open cycle--the Second Law is only applicable
to closed cycles. If the analysis is extended to the original
natural gas compressor station, thereby reconstituting a closed
cycle for the natural gas, then there is reject heat in the form of
sensible heat loss from the compressed gas, after the compressor
station
[0077] Heat addition in HEATEX2 is represented in FIG. 5 by the
section E'-G and expansion in EXP2, by the section G-H. Within the
closed air cycle, the section H-C constitutes heat rejection,
which, conventionally may be recuperated or transferred to the
incoming stream if the temperature at E is less than the
temperature at H. In this case the quantum of recuperation is the
difference in temperature between the temperature at H and
temperature at E, the remaining sensible heat (le TE-30) must be
rejected to atmosphere. In the other case, je where the temperature
at H is less than at E, all of the sensible heat represented by the
section H-C has to be rejected to atmosphere.
[0078] Returning to the flow diagram, FIG. 6, after heat addition
in HEATEX2, the air enters the second expander, EXP2, where the
pressure reduces to just above atmosphere, with attendant
production of shaft power. By virtue of the shaft interconnection
with the alternator, ALT1, this shaft power is converted to
electrical energy. In the example given, FIG. 6, a net 4630 kW is
shown as output, with consumption of 5280 kW in terms of heat
provided by the natural gas. This constitutes an overall system
thermal efficiency of 82%, in the example given. The thermal
efficiency in this system is therefore much higher than any
comparable heat driven device hitherto developed. There are two
main reasons for this:
[0079] 1) The supply of a compressed gas, which is depressurised
through means embodied in this invention and
[0080] 2) Eliminating all discharges of sensible heat to the
atmosphere (in the ideal case)
[0081] The measure of efficiency given is not strictly correct
because it does not account for the compression energy supplied at
the gas compressor station. At the locality of the gate station,
where the device embodied in this invention is implemented, the
efficiency is correct because the compression energy is supplied
without any mechanical or financial penalty.
[0082] The major independent variables enabling the output of EXP2
to be varied independently of the natural gas flow are, the air
pressure and flow at COMP1 By means of varying these parameters, a
range of power outputs may be achieved. However, a condition arises
where for any natural gas flow, for any air compression pressure, a
particular flow gives rise to the condition of no natural gas
consumption for heat input to the natural gas preheat process and
no rejected heat from the (ideal) system. This condition is called
the NGRH condition (or No Gas input or Rejected Heat
condition).
[0083] If the flow is below NGRH, for a given air compression
pressure, then an extra heat input to the natural gas flow, by
burning natural gas, is required. If the flow is above NGRH, then
some heat is rejected to the atmosphere. Mathematically, this is
represented by:
Qa<Q.sub.NGRH, RH=0, IH1>0
Qa>Q.sub.NGRH, RH>0, IH1=0
Qa=Q.sub.NGRH, RH=0, IH1=0
[0084] Here Qa is the air flow, Q.sub.NGRH is the crtical flow , RH
is the rejected heat and IH1 is the natural gas combustion heat
input to the preheat process, in HEATEX 1.
[0085] The invention is further directed to an overpressurization
Safety System. Further to the discussion of the invention in terms
of it's operating parameters, devices, modules and functions, the
invention embodies a safety device integral with the functioning
and operation of the air compressor, See FIG. 3.
[0086] When the grid is disconnected, the collapse of the back EMF
in the alternator leads to a collapse of the force which maintains
the speed constant. This leads to the RPM increasing in the
mechanical system, leading in turn to overpressurization of the
downstream circuit. The invention embodies a pressure control
system as given in FIG. 3, whereby, in addition to the compressor
air offtake which leads to the heat exchanger, at K, an additional
air release circuit on the high pressure side is provided, the
connecting pipe is indicated at G.
[0087] In the event of the grid being disconnected, the
electrically activated solenoid F, which holds up lever D when the
grid is on, is deactivated, resulting in the lever D descending
under the influence of gravity acting on weight E. This causes
rotation of the lower section of lever D, around the fulcrum
indicated at C, thereby moving lever B which is attached to a
piston within passage A. This piston is pressure balanced in that
the compressor pressure acts on both sides, the passage to the rear
(towards B) for pressure balancing is not shown. Further, the
piston attached to lever B has a clearance fit in the passage A,
facilitating eay movement.
[0088] Movement of piston attached to B shuts off the passage at K
thereby interrupting the flow of compressed air to the heat
exchanger HEATEX 1. There is pressure build up in the compressor.
The other circuit on the high pressure side now comes into
operation. Valve piston J on valve seat H lifts up at a
predetermined pressure, releasing compressed air to atmosphere. The
valve P is not only a conventional safety release valve found as
standard on any compressor, but is of the same type and with the
same or similar operating characteristic as the throttling valve in
the natural gas circuit. It has the function of modulating the
discharge pressure of the compressor, such that any tendency to
overspeed is controlled.
[0089] By means of a device to change the compression of the valve
spring, the set point of the valve may be changed to cater for any
changes in overall conditions. When the system is operating with
valve P in action, the energy released by the expansion of natural
gas in EXP 1 is dissipated by means of the expansion of compressed
air to atmosphere through valve P, which acts as a throttling
valve. In the circumstances indicated, there is interruption of the
flow to 1-IEATEX1, but in many cases, it may be desirable to keep
flow to HEATEX1 going without shutoff Under these circumstances,
both sections A-K-E and G-P-L would be connected to the air flow
pipe after HEATEXI, as shown in FIG. 4.
[0090] The system indicated constitutes a mechanical fail safe
system to prevent overpressurization of the downstream circuit, on
grid failure it acts under the influence of gravity and built in
forces to control the rate of rotation of the expander and hence
the gas throughput. The system may be used continuously, instead of
the conventional throttle valve, to control the depressurization
process.
[0091] Thus, the present invention is well adapted to carry out the
objects and attain the ends and advantages mentioned as well as
those which are inherent therein. While numerous changes may be
made by those skilled in the art, such changes are encompassed
within the spirit of this invention as defined by the appended
claims.
* * * * *