Method to Maximize LNG Plant Capacity in All Seasons
As described herein, a method and system for operating a liquefied natural gas (LNG) plant are provided. The method and system also provide for domestic natural gas production. In the present methods and systems, substantially all of the natural gas produced from a well or formation is processed to form LNG; a portion of the LNG produced is regasified; and the regasification is utilized to cool the inlet air to the gas turbines in the LNG plant, either directly or indirectly.
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This application is a divisional of U.S. patent application Ser. No. 12/982,219 filed Dec. 30, 2010 entitled “Method to Maximize LNG Plant Capacity in All Seasons”, the contents of which are herein incorporated by reference in their entirety.
FIELD OF THE INVENTIONThe present application relates to a method and system which maximizes liquefied natural gas (LNG) plant capacity in all seasons, reduces any seasonal variations in production, and provides natural gas for a domestic or local natural gas market. The method cools and condenses substantially all of a natural gas stream to produce liquefied natural gas (LNG), regasifies an amount of the LNG stream and utilizes the regasification to chill inlet air to gas turbine (GT) machines used in the refrigeration system of the LNG plant. This approach enhances the GT power output and efficiency which, in turn, increases the LNG production of the plant and reduces any seasonal variations in production. The gain in efficiency in LNG production can compensate for the cost in energy to liquefy and then regasify.
BACKGROUND OF THE INVENTIONNatural gas has become a fuel of choice for many sectors. When natural gas is produced from an indigenous field, typically the produced supply is initially separated upon collection from the inlet pipeline for the local or domestic market and for export to remote markets. The inlet gas that is destined for the local or domestic market after separation is sent as a gas to a plant or facility for cleaning and processing to be transported by gas pipeline for use as natural gas in the power sector of the local or domestic market. The inlet gas that is to be exported to remote markets is sent to a plant for liquefaction and shipped for export as LNG.
Most LNG plants utilize gas turbines to drive the refrigeration compressors required to liquefy the natural gas. Power generators are also driven by gas turbines in the LNG plants. The combined output and efficiency of all gas turbines determines the total capacity of the LNG plant. The capacity of the LNG plan is sensitively determined by the total output of the gas turbine machines. It is a challenge to maintain the power output at a stable and maximum level. One approach to achieve this goal is the installation of air conditioning devices at the air inlet so that gas turbines receive cooled stable inlet air. For example, U.S. Pat. No. 6,324,867 describes a refrigeration process. However, this approach significantly decreases the overall efficiency of the LNG plant.
With increased demand for both LNG and domestic natural gas as natural gas becomes a more important energy source, there exists a need for improving the efficiency of the processes for supplying natural gas domestically and preparing LNG for more remote markets. More efficient processes will allow LNG and natural gas to be supplied to market at competitive prices.
SUMMARY OF THE INVENTIONAs described herein, a method and system for operating a liquefied natural gas (LNG) plant are provided. The method and system also provide for domestic natural gas production. The method and system provided herein maximizes production capacity and reduces any seasonal variations in production.
In one embodiment disclosed herein is an integrated liquefied natural gas (LNG) and domestic gas production system. The system comprises (a) an inlet stream comprising natural gas; (b) a refrigeration system for reducing the temperature of the natural gas and condensing the natural gas to produce LNG; (c) a gas turbine for driving a compressor for the refrigeration system; (d) a vaporization heat exchanger for regasifying a portion of the LNG and cooling a heat transfer fluid; (e) a second heat exchanger for reducing the temperature of inlet air entering the gas turbine with the heat transfer fluid; and (f) an outlet stream comprising the regasified portion of the LNG.
In another embodiment disclosed herein is an integrated method for operating a liquefied natural gas (LNG) plant. The method comprises (i) cooling and condensing a natural gas stream in a refrigeration system to produce liquefied natural gas (LNG); (ii) operating a gas turbine to drive a compressor for the refrigeration system; (iii) regasifying a portion of the LNG; (iv) reducing the temperature of inlet air entering the gas turbine by exchanging heat with the regasified portion of the LNG directly or indirectly; and (iiv) supplying at least a portion of the regasified portion of the LNG to an outlet pipeline.
In an additional embodiment disclosed herein is a process for creating natural gas for a local gas market and liquefied natural gas (LNG) for transport. The process comprises a) cooling and condensing a natural gas stream in a LNG facility comprising a refrigeration system to produce LNG; b) regasifying a portion of the LNG; c) reducing the temperature of inlet air entering the refrigeration system by exchanging heat with the regasified portion of the LNG either directly or indirectly; d) shipping the LNG to a remote location; and e) supplying at least a portion of the regasified portion of the LNG to an outlet pipeline for the local gas market.
In the present methods and systems, substantially all of the natural gas produced from a well or formation is processed to form LNG; a portion of the LNG produced is regasified; and the regasification is utilized to cool the inlet air to the gas turbines in the LNG plant, either directly or indirectly.
DEFINITIONSIn accordance with this detailed description, the following abbreviations and definitions apply. It must be noted that as used herein, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “gas turbine” includes a plurality of such.
Unless otherwise stated, the following terms used in the specification and claims have the meanings given below:
“LNG” is liquefied natural gas. Natural gas from the well can consist of various hydrocarbons and contaminants; natural gas for the domestic market is comprised primarily of methane. At ambient temperature and pressure, LNG exists as a gas, but it can be cooled and/or pressurized to provide a liquid, which facilitates storage and transportation.
“Remote location or market” means a location that is not readily accessible or economically feasible to access by pipeline. For example, a remote location or market can be at least over a thousand miles away from the natural gas source and/or is separated in geography such that it is not accessible by pipeline, for example, separated by oceans or other large, deep bodies of water.
“Local market” means a location that is within a distance and geography from the natural gas source so that the natural gas may be supplied as a gas by pipeline. For example, local markets can be at any distance within several thousand miles from the natural gas source and is accessible by pipeline.
“Direct” in the context of heat exchange means that the heat exchange between the regasified LNG and the ambient air is direct with no intermediate heat transfer fluid involved.
“Indirect” in the context of heat exchange means that the heat exchange between the regasified LNG and the ambient air involves an intermediate heat transfer fluid. Accordingly, the temperature of inlet air entering the gas turbine is reduced by exchanging heat with the regasified portion of LNG through a heat transfer fluid system.
“Integrated” means that the steps or units of the system or interconnected so that when operating together greater efficiencies are realized in comparison to when operating independently.
“Substantially all” means at least 90% and up to 100%.
“Optional” or “optionally” means that the subsequently described event or circumstance may, but need not, occur, and that the description includes instances where the event or circumstance occurs and instances in which it does not.
When natural gas is produced from a field, typically the produced supply of natural gas is separated upon collection from the inlet pipeline, a portion is used as a supply for the local or domestic market and a portion is used as a supply to a LNG plant to be liquefied for export to remote markets. Accordingly, there are two plants required, one to prepare the natural gas for the local energy market and a separate facility for LNG production. The gas that is destined for the local or domestic market after separation is typically sent as a gas to a plant or facility for cleaning and processing to remove contaminants and then is supplied by gas pipeline for use as natural gas in the power sector of the local or domestic market. The gas that is to be exported to remote markets is sent to a separate plant for liquefaction and shipped for export as LNG.
Most LNG plants utilize gas turbines to drive the refrigeration compressors required to liquefy the natural gas. Power generators are also driven by gas turbines in the LNG plants. A portion of the natural gas collected may also be utilized as a fuel for power generation for the LNG plant, including for the gas turbines. The combined output and efficiency of all gas turbines determines the total capacity of the LNG plant.
The present application provides a method and a system which maximizes LNG plant capacity in all seasons, reduces any seasonal variations in production because of outdoor temperature changes, and also provides natural gas for a domestic or local natural gas market. In the present method and system substantially all of a natural gas stream is liquefied in a LNG facility. A portion of the LNG stream is regasified and the cooling from this process is used to reduce the temperature of inlet air entering gas turbines of the refrigeration system in the LNG plant. The regasified portion is then fed to a pipeline for use as an energy source in the domestic or local market.
Accordingly, with the present method and system, integration is utilized to increase the gas turbine power output in the LNG plant and to provide a natural gas stream suitable for the domestic or local natural gas market. A single facility is utilized to provide both the LNG and to prepare the natural gas stream for use as an energy source in the domestic power market. Thus, the present method and system provide a gain in the efficiency in both LNG production and in domestic natural gas production. The present method and system also reduce any seasonal variation in the LNG plant capacity because of outdoor temperature changes and thus ambient air temperature changes with the different seasons.
According to the present methods and system, natural gas is produced from a field or well. The produced supply of natural gas is collected. A single facility, the LNG liquefaction plant, is utilized to process substantially all of the natural gas stream, both the portion that is collected for production of LNG and for domestic or pipeline natural gas. Two separate facilities or plants (one for domestic gas production and one for LNG) are not needed or utilized according to the presently described methods and systems.
In the present methods and systems, the effect of monthly temperature variation of the location of the natural gas field or well on the capacity of the LNG plant is stabilized. The capacity of the LNG plant can be sensitive to environmental temperature variation. The capacity of the LNG plant is determined by the total output of the gas turbine machines in the refrigeration system and it is challenging to maintain the power output at a stable and maximum level.
According to the present methods and system, it has been surprisingly discovered that liquefying substantially all of the inlet natural gas stream, regasifying a portion of the LNG and using the cooling effect from regasifying to cool the inlet air for the gas turbines can maintain the power output of the LNG plant at a stable and maximum level. By making inlet air to gas turbines stable and constant throughout the entire year, the plant (or capital) utilization efficiency is also greatly improved. The gain in energy efficiency by reducing the temperature of the inlet air entering the gas turbines can compensate for the amount of energy required to produce the portion of LNG, which is later regasified. It is also beneficial to maintain the inlet air entering the gas turbines at a constant temperature in all seasons. The optimal degrees of chilling are machine-specific.
In one embodiment, the temperature of the inlet air entering the gas turbines can be reduced by 10 to 40° F. from the ambient temperature of the LNG production system. In an embodiment, the temperature of the inlet air entering the gas turbines can be reduced by at least 20° F. from the ambient temperature of the LNG plant. In another embodiment, the temperature of the inlet air entering the gas turbines can be reduced to a temperature in a range of from about 40 to 55° F. or 45 to 55° F. In an additional embodiment, the temperature of the inlet air entering the gas turbine can be reduced from an ambient temperature in a range of from about 60 to about 120° F. to a temperature in a range of from about 45 to about 55° F. In a further embodiment, the temperature of the inlet air entering the gas turbine can be reduced from an ambient temperature in a range of from about 80 to about 120° F. to a temperature in a range of from about 42 to about 60° F.
By maintaining the inlet air for the gas turbines at a constant cool temperature, the amount of power generated by the turbine remains high regardless of the ambient air temperature. By carefully regulating the LNG flow into the regas unit, it is possible to control the refrigeration supply and maintain the inlet air to a gas turbine at a cool and stable level. Thus, the gas turbine output and efficiency can be maximized in all seasons and in all climates.
For facilities in tropical regions that have high average temperatures, which relatively stable, the present system and methods can be utilized to lower the average values to maximize turbine output. In the Arctic with low temperatures and large seasonal variations, the present system and methods can be utilized to mitigate the seasonal variations. In the desert with high average temperatures and large seasonal variation, the present system and methods can be utilized to lower the average temperature and maintain the stability of the temperature.
The gain in gas turbine output and efficiency can compensate for the additional refrigeration required to produce LNG to compensate for the energy required for the initial refrigeration. The gain may be measured over the seasonal variations for climates with cold seasons and the additional production during colder seasons can be used to compensate for the additional energy required for initial refrigeration during warmer seasons.
One embodiment of the method and system is illustrated in
The inlet stream may be pre-treated if necessary in a pre-treatment unit to remove contaminants from the natural gas stream prior to liquefaction and refrigeration. The contaminants removed can include, for example, sulfur, carbon dioxide, and mercury. Prior to liquefaction and refrigeration, the inlet stream also may be chilled and any non-gaseous liquid (NGL) or heavier hydrocarbons may be separated. The non-gaseous liquid or heavier hydrocarbons may be fractionated and any gas from the fractionation can be fed to the liquefaction and refrigeration unit. From the fractionation of the non-gaseous liquid, LPG products and refrigerant make-up may also be collected.
The inlet stream of natural gas is fed to a refrigeration system for reducing the temperature of the natural gas and condensing the natural gas to produce LNG (a liquefaction and refrigeration unit (10)). The refrigeration system may be a single or multistage refrigeration system. One or more gas turbines drive a compressor for the refrigeration system. After the natural gas is liquefied, nitrogen can be removed if the nitrogen content is high. If needed, nitrogen can be removed by flashing.
The liquefied product (3) is then sent to a storage tank (30). From the storage tank, LNG is collected to ship or transport (4). As such, from the storage tank LNG can transferred to a shipping apparatus, such as a tanker or ship, for export to a remote market.
According to the present methods and system, a portion of the LNG (5) is taken from the collection/storage tank for regasification. The portion of LNG (5) is regasified in a regasification unit (40). The portion of the LNG to be regasified can be in the range of from 5% to 25% by weight of the total LNG produced. In another embodiment, the portion of the LNG to be regasified can be in the range of from 10% to 20% by weight of the total LNG produced. The regasification unit can be a vaporization heat exchanger for regasifying the portion of the LNG and cooling a heat transfer fluid. The heat transfer fluid can comprise methanol, ethanol, propane, an ethylene glycol and water mixture or any combination thereof. The heat transfer fluid can also take additional heat or refrigeration from auxiliary sources. For example, the heat transfer fluid can take additional refrigeration from a traditional refrigeration loop, the compressor of which is driven by steam generated from recovered waste heat. A second heat exchanger is utilized for reducing the temperature of inlet air entering the gas turbine with the heat transfer fluid. The second heat exchanger can comprise a cooling coil at an inlet of the gas turbine.
An example of the heat transfer fluid system is illustrated in
Thus, the portion of LNG to be regasified (5) goes through a regasification unit (40), releases its refrigeration (6) and this used to reduce the temperature of inlet air entering gas turbines (60) of the refrigeration system in the LNG plant either directly or indirectly. As such, the temperature of inlet air is reduced by exchanging heat either directly or indirectly with the regasified portion of the LNG. In one embodiment, the heat is exchanged indirectly through the use of an intermediate heat transfer fluid. It may also be used to reduce the temperature of inlet air entering the power generation unit (50) of the LNG plant. The intermediate heat transfer fluid may be used to take refrigeration from the regasified portion of the LNG to other auxiliary sources. For example, the heat transfer fluid can take additional refrigeration to a traditional refrigeration loop, the compressor of which is driven by steam generated from recovered waste heat.
In an embodiment, a portion of the LNG to be regasified can also be used to sub-cool a refrigerant in at least one stage of the refrigeration system by heat exchange. In one embodiment, from 10% to 90% by weight of the regasified portion of the LNG is used to sub-cool a refrigerant in at least one stage of the refrigeration system.
The regasified LNG (7) is then supplied to a natural gas pipeline for domestic or local natural gas production. By taking the inlet natural gas stream (1) and creating LNG and then using the regasified LNG as the stream for the local natural gas market, the natural gas stream for the local market has any contaminants removed by the LNG process. Therefore, a separate facility is not required to remove contaminants, such as sulfur and carbon dioxide, from the natural gas before providing it to the domestic energy market. The regasified LNG has substantially all contaminants removed by the LNG process.
If the regasified portion of the LNG is not needed for domestic natural gas production, or it is not all needed, the regasified LNG, or a portion of the regasified LNG, can be recycled to the refrigeration system to provide LNG.
The regasified LNG does not require an additional separate cleaning facility prior to use as domestic natural gas because it is cleaned sufficiently in the liquefaction process. Accordingly, the regasified portion of the LNG can be exported directly by pipeline for use in a local or domestic natural gas market. Because the regasified LNG has substantially all contaminants removed by the LNG process, the regasified LNG is cleaner than natural gas typically recovered for local or domestic market production. Natural gas recovered for local or domestic market production is processed to remove contaminants, such as sulfur and carbon dioxide, to meet pipeline specifications.
The regasified LNG can also be used to blend with natural gas directly collected for domestic production to meet pipeline specifications. When the regasified LNG is blended with natural gas, the natural gas can be processed less severely removing fewer contaminants so that the natural gas alone would not meet pipeline specifications. But the blend can meet pipeline specifications. The regasified LNG can be blended with natural gas, which has been not processed or has been processed less severely, and the blend meets pipeline specifications. For example, in cooler seasons it may be possible to blend high purity, regasified LNG with unprocessed or less processed natural gas and meet pipeline specifications for domestic gas production.
The gas turbine of the refrigeration compressors condenses water vapor (8) from the ambient air of the facility. The power generator for the LNG plant may also condense water vapor (8) from the ambient air. This condensed water (8) is distilled quality water. Accordingly, the condensed water (8) can be collected and used for other uses in the plant. For example, it can be used for wet compression, evaporative cooling, and/or fogging the inlet air to the gas turbines. The water can be used as plant process water such as hydrogen sulfide removal by passing natural gas through water or amine based solution. The water can be used for compressor circulation cooling and inlet humidity adjustment. Because it is distilled quality water, it can also be used for any use for which distilled water may be needed in the area of the plant. For example, it can be used as a source for drinking water or irrigation water as well. A source for drinking water or irrigation water may be particularly useful in desert locations.
Accordingly, with the present method and system, integration is utilized to increase the gas turbine power output in the LNG plant and to provide a natural gas stream suitable for the domestic or local natural gas market. A single facility is utilized to provide both the LNG and to prepare the natural gas stream for use in the domestic power market. Thus, the present method and system provides a gain in the efficiency in both LNG production and in domestic natural gas production.
The gain in energy efficiency by reducing the temperature of the inlet air entering the gas turbines can compensate for the amount of energy required to produce the portion of LNG, which is later regasified. In one embodiment, the efficiency of the gas turbines is increased by at least 3%. The efficiency may be increased by at least 3% by reducing the temperature of the inlet air from an ambient temperature of 90° F. to a temperature of 50° F.
In one embodiment, the temperature of the inlet air entering the gas turbines can be reduced by 10 to 40° F. from the ambient temperature of the LNG production system. In an embodiment, the temperature of the inlet air entering the gas turbine is reduced by at least 20° F. from the ambient temperature of the LNG plant. In an embodiment, the temperature of the inlet air entering the gas turbines can be reduced to a temperature in a range of from about 40 to 55° F. In an embodiment, the temperature of the inlet air entering the gas turbines can be reduced to a temperature in a range of from about 45 to 55° F.
In one embodiment, the temperature of the inlet air entering the gas turbine can be reduced from an ambient temperature in a range of from about 60 to about 120° F. to a temperature in a range of from about 45 to about 55° F. In an additional embodiment, the temperature of the inlet air entering the gas turbine can be reduced from an ambient temperature in a range of from about 80 to about 120° F. to a temperature in a range of from about 42 to about 60° F.
In the methods as disclosed herein, a single liquefied natural gas plant is utilized to create natural gas for a local gas market and liquefied natural gas for transport to a remote market. The process comprises cooling and condensing a natural gas stream in a LNG facility comprising a refrigeration system to produce LNG. One or more gas turbines are used to operate compressors for the refrigeration system of the LNG plant. A portion of the LNG is taken to be regasified. The temperature of inlet air entering the gas turbines of the refrigeration system is reduced by exchanging heat with the portion of the LNG to be regasified either directly or indirectly. In certain embodiments, an intermediate heat transfer fluid is used to take refrigeration from the portion of the LNG to be regasified an cool the inlet air. The LNG produced from the facility is shipped to remote markets and at least a portion of the regasified LNG is supplied to an outlet pipeline for local gas markets.
In the presently disclosed methods and system, the improvement comprises converting substantially all of the produced natural gas stream from a well or field to LNG. A portion of the LNG is then regasifying a portion for domestic gas production. In regasifying a portion of the LNG, the regasification process is used to chill inlet air to gas turbines used in the refrigeration system of the LNG plant. This approach enhances the GT power output and efficiency which, in turn, increases the LNG production of the plant. The gain in efficiency in LNG production can compensate for the cost in energy to liquefy and then regasify. Accordingly, a single facility is used to process the natural gas produced from the formation.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope thereof.
Claims
1. An integrated method for operating a liquefied natural gas (LNG) plant, the method comprising:
- (i) cooling and condensing a natural gas stream in a refrigeration system to produce liquefied natural gas (LNG);
- (ii) operating a gas turbine to drive a compressor for the refrigeration system;
- (iii) regasifying a portion of the LNG;
- (iv) reducing the temperature of inlet air entering the gas turbine by exchanging heat with the regasified portion of the LNG directly or indirectly; and
- (iiv) supplying at least a portion of the regasified portion of the LNG to an outlet pipeline.
2. The method of claim 1, wherein a gain in energy efficiency by reducing the temperature of the inlet air entering the gas turbine compensates for an amount of energy required to produce the portion of the LNG regasified in step (iii).
3. The method of claim 1, wherein the temperature of inlet air is reduced by exchanging heat indirectly with the regasified portion of the LNG using an intermediate heat transfer fluid.
4. The method of claim 1, wherein the portion of the LNG regasified is in a range from 5% to 25% by weight.
5. The method of claim 1, wherein the temperature of the inlet air entering the gas turbine is reduced by at least 20° F. from the ambient temperature of the LNG plant.
6. The method of claim 1, wherein the temperature of the inlet air entering the gas turbine is reduced from an ambient temperature in a range from about 80 to about 120° F. to a temperature in a range from about 42 to about 60° F.
7. The method of claim 1, wherein the temperature of the inlet air entering the gas turbine is reduced by 10 to 40° F. from the ambient temperature of the LNG production system.
8. The method of claim 1, wherein the temperature of the inlet air entering the gas turbine is reduced from an ambient temperature in a range from about 60 to about 120° F. to a temperature in a range from about 45 to about 55° F.
9. The method of claim 1, wherein the temperature of the inlet air entering the gas turbine is reduced to a temperature in a range from about 45 to about 55° F.
10. The method of claim 1, wherein an efficiency of the gas turbine is increased by at least 3% by reducing the temperature of the inlet air from 90 to 50° F.
11. The method of claim 1, further comprising a step of supplying the regasified portion of the LNG to a natural gas pipeline for domestic gas production.
12. The method of claim 1, further comprising a step of recycling the regasified portion of the LNG to the natural gas stream in step (i).
13. The method of claim 1, further comprising a step of sub-cooling a refrigerant in at least one stage of the refrigeration system by heat exchanging with from 10% to 90% by weight of the regasified portion of the LNG.
14. The method of claim 1, further comprising a step of pre-treating the natural gas stream prior to step (i) to remove contaminants from natural gas stream.
15. The method of claim 1, further comprising a step of fractionating the natural gas stream after a first stage in the refrigeration system to remove heavier hydrocarbons from the natural gas stream.
16. The method of claim 1, further comprising a step of collecting water condensed from reducing the temperature of the inlet air in step (iv).
17. The method of claim 16, further comprising supplying the water as drinking water or irrigation water a local area.
18. The method of claim 1, further comprising a step of using the regasified LNG supplied to the outlet pipeline to supply a domestic natural gas market.
19. The method of claim 1, further comprising a step of blending the regasified LNG with natural gas to provide a blended gas meeting pipeline specifications for domestic gas production.
20. A process for creating natural gas for a local gas market and liquefied natural gas (LNG) for transport comprising:
- a) cooling and condensing a natural gas stream in a LNG facility comprising a refrigeration system to produce LNG;
- b) regasifying a portion of the LNG;
- c) reducing the temperature of inlet air entering the refrigeration system by exchanging heat with the regasified portion of the LNG either directly or indirectly;
- d) shipping the LNG to a remote location; and
- e) supplying at least a portion of the regasified portion of the LNG to an outlet pipeline for the local gas market.
21. The method of claim 20, further comprising a step of collecting water condensed from reducing the temperature of the inlet air in step (c).
22. The method of claim 21, further comprising supplying the water as drinking water or irrigation water a local area.
23. The method of claim 20, further comprising a step of blending the regasified LNG with natural gas to provide a blended gas meeting pipeline specifications for domestic gas production.
Type: Application
Filed: May 30, 2014
Publication Date: Sep 18, 2014
Applicant: Chevron U.S.A. Inc. (San Ramon, CA)
Inventors: Robert C. Havlik (Montgomery, TX), Stanley H. Huang (Sugar Land, TX)
Application Number: 14/292,336
