U.S. patent application number 13/127704 was filed with the patent office on 2011-09-01 for system for combined cycle mechanical drive in cryogenic liquefaction processes.
This patent application is currently assigned to Hamworthy Gas Systems AS. Invention is credited to Bjorn Harald Haukedal, Lars Horlyk.
Application Number | 20110209496 13/127704 |
Document ID | / |
Family ID | 42153432 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110209496 |
Kind Code |
A1 |
Horlyk; Lars ; et
al. |
September 1, 2011 |
SYSTEM FOR COMBINED CYCLE MECHANICAL DRIVE IN CRYOGENIC
LIQUEFACTION PROCESSES
Abstract
A system for producing liquefied and sub-cooled natural gas by
means of a refrigeration assembly using a single phase gaseous
refrigerant comprises at least two expanders; a compressor
assembly; a heat exchanger assembly for heat absorption from
natural gas; and a heat rejection assembly, in which the expanders
are arranged in expander loops and the refrigerant to the
respective expander is served in a compressed flow by means of the
compressor assembly having compressors or compressor stages
enabling adapted inlet and outlet pressures for the respective
expander. According to the present the expanders and compressors
assembly are assembled in two mechanically connected compressor and
expander packages of which one is driven by a gas turbine and the
other is driven by a steam turbine, the steam primarily being
generated by exhaust gases from the gas turbine in a waste heat
recovery unit, and in that the expanders and compressors assemblies
are distributed between the two compressor and expander packages to
optimize the steam utilization and to balance the power generated
by the gas turbine and the steam turbine.
Inventors: |
Horlyk; Lars; (Sandvika,
NO) ; Haukedal; Bjorn Harald; (Bekkestua,
NO) |
Assignee: |
Hamworthy Gas Systems AS
Asker
NO
|
Family ID: |
42153432 |
Appl. No.: |
13/127704 |
Filed: |
October 16, 2009 |
PCT Filed: |
October 16, 2009 |
PCT NO: |
PCT/NO2009/000362 |
371 Date: |
May 17, 2011 |
Current U.S.
Class: |
62/613 |
Current CPC
Class: |
F25J 2240/82 20130101;
F25J 1/0289 20130101; F25J 1/029 20130101; F25J 1/0298 20130101;
F25J 1/0283 20130101; F25J 1/0207 20130101; F25J 1/0022 20130101;
F25J 1/0288 20130101; F25J 2270/16 20130101; F25J 2230/20 20130101;
F25J 1/0242 20130101; F25J 1/0279 20130101; F25J 1/005 20130101;
F25J 1/0282 20130101 |
Class at
Publication: |
62/613 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 4, 2008 |
NO |
20084656 |
Claims
1. A system for producing liquefied and sub-cooled natural gas by
means of a refrigeration assembly using a single phase gaseous
refrigerant comprising: at least two expanders; a compressor
assembly; a heat exchanger assembly for heat absorption from
natural gas; and a heat rejection assembly, in which the expanders
are arranged in expander loops and the refrigerant to the
respective expander is served in a compressed flow by the
compressor assembly having compressors or compressor stages
enabling adapted inlet and outlet pressures for the respective
expander, wherein the expanders and compressors or their stages are
assembled in two mechanically connected compressor and expander
packages, of which one is driven by a gas turbine and the other is
driven by a steam turbine, the steam primarily being generated by
exhaust gases from the gas turbine in a waste heat recovery unit,
and wherein the expanders and compressors or their stages are
distributed between the two compressor and expander packages to
optimize the steam utilization and to balance the power generated
by the gas turbine and the steam turbine.
2. A system according to claim 1, wherein a duct burner arranged in
the waste heat recovery unit or another heat source compensates for
smaller deficits in steam demand as to optimize the process.
3. A system according to claim 1, wherein each of the gas and steam
turbines are mechanically coupled to an integrated gear box
comprising different combinations of compressor and expander
units.
4. A system according to claim 1, wherein each expander loop is
part of fluidly separated refrigerant cycles, wherein all
compressors and expanders are associated with at least one of the
refrigerant cycles being arranged on the gearbox directly driven by
the gas turbine so that cooling duty can be established and
controlled independent of the status of the steam system.
5. A system according to claim 1, wherein each of the gas and steam
turbines are mechanically coupled to individual compressor
sections, the expanders being arranged on separate stand-alone
expander-booster-compressor skids.
Description
BACKGROUND OF THE INVENTION
[0001] Different variations of the Brayton cooling cycle have been
continuously developed and described in patent literature over the
years. The nature of the first single expander systems could speak
in favour of utilizing the process for cooling applications, rather
than for liquefaction, due to the fact that the refrigerant remains
in single phase throughout the cycle and therefore is easier
adaptable to single phase loads like gas cooling, for instance.
[0002] However, even though the traditional Brayton process did not
perform as well in terms of liquefaction power as cascade
processes, mixed refrigerant processes and other large scale
liquefaction systems, the simplicity, robustness and flexibility of
the process resulted in some popularity also for liquefaction
purposes, e.g. for the production of liquefied natural gas (LNG).
Especially for smaller plant the simple Brayton systems have proved
to be feasible.
[0003] When regarded for use in larger scale natural gas
liquefaction facilities, most factors mentioned above have been
considered to be positive side effects of the Brayton loop, but the
specific energy demand has overshadowed the other favourable
effects compared to other technologies. Recent developments of the
Brayton cycle like those described in Norwegian Patent Application
2008 3740, for instance, has narrowed the gap between the Brayton
cycles and other technologies also in terms of energy performance.
In addition to new requirements for more generic designs, safer
processes and other aspects, these have contributed to revitalize
variants of the Brayton loop for new medium to large scale LNG
applications, e.g. in the floating LNG market.
[0004] A characteristic for most of the different process variants
described in Norwegian Patent Application 2008 3740 is the large
number of compression and expansion stages required. At first
glance, this seems like a complicating factor when utilizing the
processes for an application wherein equipment weight and
dimensions need to be minimized. As shown hereinafter, this is not
the fact.
[0005] Main objects of present invention are to provide for a
solution establishing compact mechanical layouts, a low specific
power demand, and a unique and optimal balancing of power consumers
and suppliers for such processes. Another aspect is how to combine
such mechanical layout in a cogeneration plant by direct mechanical
drives for the compressor and/or expander units.
[0006] Thus, the system is utilising the energy balance in a dual
or trippel Brayton cycle as to be optimized and flexible, as well
as energy efficient. The system also facilitates a power balance
that is dynamic between the different loops, i.e. low temperature,
medium temperature and high temperature loop, involving the process
lay out can handle fluctuations and varying conditions.
SUMMARY OF THE INVENTION
[0007] According to the present invention these object are achieved
by a system for producing liquefied and sub-cooled natural gas by
means of a refrigeration assembly using a single phase gaseous
refrigerant comprising at least two expanders; a compressor
assembly; a heat exchanger assembly for heat absorption from
natural gas; and a heat rejection assembly, in which the expanders
are arranged in expander loops and the refrigerant to the
respective expander is served in a compressed flow by means of the
compressor assembly having compressors or compressor stages
enabling adapted inlet and outlet pressures for the respective
expander, wherein the expanders and compressors or their stages are
assembled in two mechanically connected compressor and expander
packages of which one is driven by a gas turbine and the other is
driven by a steam turbine, the steam primarily being generated by
exhaust gases from the gas turbine in a waste heat recovery unit,
and in that the expanders and compressors or their stages are
distributed between the two compressor and expander packages to
optimize the steam utilization and to balance the power generated
by the gas turbine and the steam turbine.
[0008] Further embodiments of the present invention are specified
in the dependent patent claims and the detailed description
below.
[0009] Briefly, the present invention is involving a development of
the double or triple Brayton loops as depicted by Norwegian Patent
Application 2008 3740. The ambition is higher efficiency, i.e. less
demand for kW per kg of the LNG produced, being an essential factor
in today's operation to liquefy natural gas. The result of
addressing this issue is a huge leap in the single refrigerant
systems. The invention is unique in the respect that a gas turbine
is providing the net power demand for a setup of at least one
compressor and possibly at least one expander unit, and the exhaust
gas from the gas turbine is used to generate steam. The steam is
then routed to a steam turbine, driving another setup of at least
one compressor and possibly at least one expander unit.
[0010] The low temperature or sub-cooling loop is physically placed
on the gas turbine driven unit and the high temperature or
de-superheating loop is preferably physically situated on the steam
turbine driven unit. The medium temperature or condensation and
cooling loop is typically split between the two. Since the level of
generated power between the steam turbine and the gas turbine is in
order of magnitude 2:1, this system layout is well suited for a
process solution having dual or triple Brayton loops. This results
in a setup with extremely low fuel demand compared to normal gas
turbine driven systems without waste heat recovery units, and is
hence economical, environmentally friendly and has low operating
cost.
[0011] Via either a mechanical device such as a clutch, gear or
other means, the system can be power balanced to fulfil the
required power consumption of the total system, i.e. if any gas
composition is such that thermodynamical alterations are
insufficient to fulfil the power requirements, this can be done via
a mechanical transfer instead. Otherwise this is normally achieved
by process adjustments such as pressure levels, or via guide vanes
in the compressor or expanders.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 shows a schematic lay out of a triple Brayton cycle
as described in Norwegian Patent Application 2008 3740; and
[0013] FIG. 2 is a schematic lay out of the compressor and expander
driver setup based on the triple Brayton cycle in FIG. 1 including
a gas and steam turbine.
DETAILED DESCRIPTION OF THE INVENTION
[0014] As illustrated in FIG. 1, the present refrigeration assembly
comprises three different refrigeration loops: [0015] a high
temperature loop, here depicted with two compressors 12, 13 or
compressor stages and one expander 1 or expander stage, hereinafter
only denoted compressors and expanders, respectively. As for all
three loops this is a base case, but if the process advantages are
obvious, this lay out may consist of a different number of
compressor or expander units, depending process requirements or
other criteria. The high temperature loop is used for
de-superheating; [0016] a medium temperature loop is used to
condense the LNG, and this loop normally consists of two
compressors 14, 15 and one expander 2; and [0017] a low temperature
loop consists of three compressors 16, 17, 18 and one expander in
the base case also, and is used to sub-cool the LNG.
[0018] Further information for the refrigeration assembly is to be
found in the Norwegian Patent Application 2008 3740 and the same
publication is incorporated here by reference for all relevant
purposes.
[0019] Steam turbines based on waste heat recovery from gas
turbines are well known in the process and power generation
industries. In most cases at least one; cf. U.S. Pat. No.
6,640,586--often both--of the turbines in such a combined power
generation unit are used for electrical power generation as the
drivers are directly connected to an electrical generator.
[0020] The present invention describes a system where the two
drivers of a combined cycle system in the form of a gas and steam
turbine are used as direct mechanical drivers for different
rotating machinery, i.e. compressors, expanders, gears, etc.
[0021] As already mentioned, the processes described by the
Norwegian Patent Application 2008 3740 comprise at least two
expander loops and an undefined number of compressors or compressor
stages. Depending on the preferred process layout, especially in
cases with one dedicated closed Brayton cycle for each expander
loop, the number of compression and/or expansion stages can be
quite high, see FIG. 1. This can be solved by employing a multiple
number of drivers, but often a more compact design is mandatory due
to weight and footprint limitations.
[0022] Integrally geared machines offers the potential of combining
several compression and expansion stages on one common gearbox 205,
305, involving energy supply and consumption from the expanders and
compressors, respectively. The number of stages capable of being
integrated on one bullgear, i.e. one large toothed gear 206, 306
which in turn drives compressor or expander units attached via
pinion gears 207, 307, are limited by the physical geometry of the
impellers or volutes and size of the gear itself. Integrally geared
machines normally combine two impellers for each pinion shaft
rotating at the same speed. For normal integrally geared machines
the number of pinion axes is limited to three, maximum four, which
consequently allows for a maximum of six to eight impellers.
Sometimes and depending on the drive speed the drivers themselves
can also be connected to a pinion axis instead of the bull gear
itself. This fact limits the maximum number of impellers per bull
gear further compared to the number specified above.
[0023] In the process illustrated by FIG. 1 a total number of ten
impellers is required, i.e. in any case more than capable of being
connected to one single bullgear. In this case at least one further
bullgear is likely to be introduced. FIG. 2 shows one mechanical
setup of the process depicted by FIG. 1 in which the primary driver
of each bullgear is a gas turbine and a steam turbine,
respectively.
[0024] As a rule of thumb, in a combined cycle gas turbines system
the maximal shaft power of a steam turbine is about 50% of the gas
turbine. Due to the large number of impellers a discretization of
the process is possible enabling at least one process cycle to be
driven directly by the gas turbine, and at least one further to be
split between the gas turbine bullgear and the steam turbine
bullgear in order to approximate the 2:1 power ratio between the
gas and steam turbines, respectively.
[0025] In FIG. 1 a process with three fluidly separated Brayton
cycles are shown. A possible way of discretizing the process is
indicated by FIG. 2 where the entire low-temperature Brayton cycle
associated with expander 3 is connected to the gas turbine bull
gear, the entire high temperature Brayton cycle including the
expander 1 is connected to the steam turbine bull gear, and the
intermediate temperature Brayton cycle comprising the expander 2 is
split between the two bull gears.
[0026] In FIG. 2 a possible setup of the packages 200, 300
including a gas turbine 201 and steam turbine 301, respectively.
The gas turbine package is including the low temperature expander
3, and the low temperature compressors 16, 17, 18 and medium
temperature compressor 15 whereas the steam turbine package is
comprising the medium and high temperature expanders 1, 2, and the
medium temperature compressor 14 and the high temperature
compressors 12, 13.
[0027] Such arrangement allows for starting up the low temperature
loop independently of the other Brayton cycles assuming the
intermediate temperature Brayton cycle compressor connected to the
gas turbine bull gear is fully recycled. The main heat exchanger 8
or cold box can then be cooled down simultaneously as the steam
generation is initiated. The mechanical lay out also allows for a
coupling between the two driven integrally geared compressors via a
clutch, shaft or gearbox, so that unbalance in the power delivered
from the drivers and power demand from the integrally geared
compressors can be handled.
[0028] The solution indicated above opens for the possibility of
using the steam generated in the gas turbine directly in the steam
turbine, at an optimum power generation and distribution and with a
low specific power consumption. Normally the steam in a
cogenerative setup is sent to a steam turbine, which drives a
generator which in turn feeds an electric motor, with all the
inherent energy losses. This system utilises the energy directly,
and also makes it possible to make a mechanical lay out that is
small in footprint, weight and cost. The steam is generated via
coils in the exhaust stack in the gas turbine, and routed through
the steam turbine at typically two pressure levels, one overheated
high pressure steam level, and one medium pressure steam level.
[0029] The gas turbine has a waste heat recovery unit 202. The
waste heat recovery unit is in principle a closed loop circulation
consisting of a coil in the exhaust stack wherein the steam is
generated by the excess heat in the exhaust. The steam therefrom
this is directly utilized in the steam turbine. The two integrally
geared compressor and expander setups may also be mechanically
coupled via a clutch, shaft, or gear. A duct burner arranged in the
waste heat recovery unit 202, not illustrated, may compensate for
smaller deficits in steam demand whereby the process is optimized.
As an alternative it is possible to use another suitable heat
source not included in the heat recovery unit.
[0030] Each of the gas and steam turbines 201, 301 can mechanically
be coupled to individual compressor sections having the expanders
1, 2, 3 arranged on separate stand-alone
expander-booster-compressor skids, not shown in the drawings.
* * * * *