Patent Application
20110289941
The subject matter disclosed herein relates generally to regasification of Liquefied Natural Gas (LNG), and more specifically to methods and systems utilizing Brayton cycles for regasification of LNG.
Conventionally, natural gas is transported in a liquefied form, that is, as LNG, which is subsequently regasified for distribution as pipeline natural gas, or for combustion use. LNG is typically transported at a temperature of about 160 degrees Celsius below zero, at a pressure of about 1 to 2 bar, and needs to be regasified before consumption or distribution to a temperature between about 10 degrees Celsius and about 30 degrees Celsius and a pressure between about 30 bar and about 250 bar.
Certain conventional techniques use seawater as a heat source for the regasification of LNG, which use, may under certain circumstances, have a negative impact on the environment. For example, cooling of sea water using a LNG regasification process involving seawater as a heat source may produce unforeseen effects on marine life and the ecosystem in the immediate neighborhood of the LNG regasification installation. Among other conventional techniques, natural gas may be combusted to produce the heat needed for the regasification of LNG, which increases the carbon footprint of the LNG use, for example, for power generation.
Accordingly, a need exists for an improved method and apparatus for regasification of LNG that overcome at least some of the abovementioned problems associated with conventional LNG regasification techniques.
According to an embodiment of the present invention a power plant including an apparatus for regasification of liquefied natural gas (LNG) includes a compressor configured to pressurize a working fluid, a heat recovery system configured to provide heat to the working fluid, a turbine configured to generate work utilizing the working fluid, and one or more heat exchangers configured to transfer heat from the working fluid. The heat exchanger is configured to transfer heat to a first stage liquefied natural gas at a first pressure, and at least one of a second stage liquefied natural gas at a second pressure and the compressed working fluid.
According to another embodiment of the present invention, a method for regasification of liquefied natural gas in a LNG power generation plant includes recovering heat from a topping cycle of the power generation plant and heating a working fluid of a bottoming cycle of the power generation plant to provide a heated working fluid. At least a portion of the energy of the heated working fluid is released to generate work. Heat from the working fluid after generating work is transferred to a first stage liquefied natural gas at a first pressure, and at least one of a second stage liquefied natural gas at a second pressure and a compressed working fluid.
According to another embodiment of the present invention, a method for retrofitting an apparatus for regasification of liquefied natural gas in a LNG power generation plant includes providing one or more heat exchangers configured to transfer heat from a working fluid to a first stage liquefied natural gas at a first pressure and at least one of a second stage liquefied natural gas at a second pressure and a compressed working fluid. At least one of a first stage LNG pump configured to provide the first stage liquefied natural gas at the first pressure, and at least one second stage LNG pump configured to provide a second stage liquefied natural gas at the second pressure is also provided. The one or more heat exchangers, the first stage LNG pump, and the second stage LNG pump form a part of a modified bottoming Brayton cycle of the LNG power generation plant.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As used herein, an element or function recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or functions, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the claimed invention should not be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.
As noted, in one embodiment, the present invention provides a power plant including an apparatus for regasification of liquefied natural gas (LNG), the apparatus comprising (a) a compressor configured to pressurize a working fluid; (b) a heat recovery system configured to provide heat to the working fluid; (c) a turbine configured to generate work utilizing the working fluid; and (d) one or more heat exchangers configured to transfer heat from the working fluid to a first stage liquefied natural gas at a first pressure, and at least one of a second stage liquefied natural gas at a second pressure and a compressed working fluid.
In various embodiments, the power plant comprises a first stage LNG pump which may be used to provide a first stage liquefied natural gas at the first pressure, and a second stage LNG pump to provide the second stage liquefied natural gas at the second pressure.
A working fluid is used to capture heat generated by the power plant and transfer it in stages to the LNG being regasified. In various embodiments, the working fluid is heated in a heat recovery system configured to provide heat to the working fluid. In one embodiment, the working fluid is heated in the heat recovery system to a temperature between about 300° C. and about 700° C. In one embodiment, the heat recovery system is configured to extract heat from the hot exhaust gases produced by a power generation turbine. In an alternate embodiment, the heat recovery system is configured to extract heat from an external thermal cycle. In one embodiment, the external thermal cycle is a topping cycle of a LNG power generation plant.
In various embodiments, transfer of heat from the working fluid to the LNG is conducted in a heat exchanger. In one embodiment, the heat exchanger is configured to provide a heated first stage liquefied natural gas at a temperature between about −140° C. and about −110° C.
In one embodiment, the heat exchanger is configured to receive a second stage liquefied natural gas at a temperature between about −130° C. and about −100° C. and a pressure between about 50 bar and about 700 bar. In one embodiment, the heat exchanger is configured to provide a heated second stage liquefied natural gas at a temperature between about 0° C. and about 40° C.
In one embodiment, at least two heat exchangers, a first heat exchanger and a second heat exchanger are present. In one such embodiment, the first heat exchanger is configured to provide a heated first stage liquefied natural gas, and the second heat exchanger is configured to provide a heated second stage liquefied natural gas.
In one embodiment, the heat exchanger is configured to transfer heat to the second stage liquefied natural gas and the compressed working fluid. In one embodiment, the compressed working fluid is delivered to the heat exchanger at a temperature between about −30° C. and about 50° C. and a pressure between about 100 bar and about 200 bar. Under such circumstances the heat exchanger may be said to be configured to receive the compressed working fluid at a temperature between about −30° C. and about 50° C. and a pressure between about 100 bar and about 200 bar.
In one embodiment, the present invention provides a method for regasification of liquefied natural gas in a LNG power generation plant, the method comprising: (a) recovering heat from a topping cycle of the power generation plant and heating a working fluid of a bottoming cycle of the power generation plant to provide a heated working fluid; (b) releasing at least a portion of the energy contained in the heated working fluid to generate work; and (c) transferring heat from the working fluid after generating work to a first stage liquefied natural gas at a first pressure, and at least one of a second stage liquefied natural gas at a second pressure and a compressed working fluid.
In one embodiment, the method employs a working fluid selected from the group consisting of argon, helium, carbon dioxide, and nitrogen. In an alternate embodiment, the method employs a working fluid comprising at least one of argon, helium, carbon dioxide, and nitrogen. In one embodiment, the working fluid is nitrogen.
In one embodiment, the working fluid is heated in a heat recovery system associated with the topping cycle of the power generation plant to temperature in a range from about 300° C. to about 700° C. In an alternate embodiment, the working fluid is heated in a heat recovery system associated with the topping cycle of the power generation plant to temperature in a range from about 350° C. to about 650° C. In yet another embodiment, the working fluid is heated in a heat recovery system associated with the topping cycle of the power generation plant to temperature in a range from about 400° C. to about 600° C.
In one embodiment of the method of the present invention, the first stage liquefied natural gas has a temperature between about −160° C. and about −140° C. and a pressure of from about 1 bar to about 50 bar. In an alternate embodiment, the first stage liquefied natural gas has a temperature between about −160° C. and about −140° C. and a pressure of from about 2 bar to about 15 bar.
In one embodiment of the method of the present invention, the first stage liquefied natural gas is introduced into a heat exchanger where it absorbs heat from the working fluid to provide on emerging from the heat exchanger a heated first stage liquefied natural gas having a temperature between about −140° C. and about −110° C.
In one embodiment of the method of the present invention, the second stage liquefied natural gas is introduced into a heat exchanger at a temperature between about −130° C. and about −100° C. and a pressure between about 50 bar and about 700 bar. The second stage liquefied natural gas absorbs heat from the working fluid being introduced into the heat exchanger to provide on emerging from the heat exchanger a heated second stage liquefied natural gas having a temperature between about 0° C. and about 40° C.
In one embodiment of the method of the present invention, heat is transferred from the working fluid to the first stage liquefied natural gas in a first heat exchanger, and from the working fluid to the second stage liquefied natural gas in a second heat exchanger, to provide a heated first stage liquefied natural gas and a heated second stage liquefied natural gas.
In one embodiment of the method of the present invention, a single heat exchanger is used to transfer heat from the working fluid to the first stage liquefied natural gas and the second stage liquefied natural gas. Thus, heat is transferred from the working fluid to the first stage liquefied natural gas in a first heat exchanger, and from the working fluid to the second stage liquefied natural gas in the same first heat exchanger to provide a heated first stage liquefied natural gas and a heated second stage liquefied natural gas.
As noted, in one embodiment of the method of the present invention, heat is recovered from a topping cycle of a power generation plant and is used to heat a working fluid of a bottoming cycle of the power generation plant to provide a heated working fluid. The working fluid may be heated in a heat recovery system integrated into the power generation plant. Typically, the working fluid is introduced into a heat exchanger at a point downstream of an energy extraction device, such as a turbine which uses a portion of the energy contained in the heated working fluid to generate work. In one embodiment, the working fluid is introduced into a heat exchanger at a point downstream of an energy extraction device and transfers heat to the first stage liquefied natural gas to provide a heated first stage liquefied natural gas. The working fluid emerging from the heat exchanger may thereafter be subjected to a compression step to provide a compressed working fluid. Additional heat may be extracted from this compressed working fluid by passing the compressed working fluid through one or more heat exchangers in contact with either or both of the first stage liquefied natural gas and the second stage liquefied natural gas. In one embodiment, the temperature of the compressed working fluid is sufficiently low such that heat is transferred to the compressed working fluid as it passes through the heat exchanger. Under such circumstances, the heat exchanger is said to be configured to transfer heat to the compressed working fluid. In one embodiment, the compressed working fluid is introduced into the heat exchanger at a temperature between about −30° C. and about 50° C. and a pressure between about 100 bar and about 200 bar.
The bottoming cycle 132 includes a turbine 114 for generating work from the working fluid, a heat exchanger 118 to transfer heat from the working fluid to LNG for regasification, and a compressor 116 to pressurize the working fluid. In the illustrated embodiments, the working fluid of the bottoming cycle is any suitable fluid which is relatively inert under normal circumstances, and may be selected to mitigate fire, explosion, or other safety hazards. Suitable working fluids include but are not limited to generally inert gases such as, argon, helium, nitrogen, carbon dioxide among others. While in the embodiments discussed herein, nitrogen is the working fluid intended, those skilled in the art will readily appreciate that alternate working fluids generally known in the art are usable within the scope and spirit of the present invention. The system 100 further comprises a first stage LNG pump for providing a first stage liquefied natural gas to the heat exchanger 118, and a second stage LNG pump for providing a second stage liquefied natural gas to the heat exchanger 118. As illustrated by
Still referring to the embodiment illustrated in
In summary, the 3-stream heat exchanger 118 operates by having the first stage liquefied natural gas pumped to an intermediate pressure (advantageously as low as possible) and sent to the first stage LNG stream 142 at a very low temperature. The first stage liquefied natural gas absorbs heat from the working fluid and exits the first stage LNG stream 142 in a liquid state. This liquefied natural gas emerging from the heat exchanger is then pumped to a higher pressure (second stage), and is reintroduced into the heat exchanger 118 as the second stage LNG stream 144 to be fully vaporized by a second thermal contact with the working fluid which has a relatively high temperature (around 50-250° C. as the working fluid emerges from the turbine) relative to the liquefied natural gas being treated. However, those skilled in the art will appreciate that the concepts described herein with respect to the various illustrations are not restricted to a 3-stream heat exchanger such as 118, and include other variations such as those will occur readily to those skilled in the art. For example, according to an embodiment (further described with respect to
It has been discovered that decreasing the minimum temperature of the working fluid employed has a beneficial effect on the overall efficiency of the LNG liquefication process and raises the electrical efficiency of the bottoming cycle. In an embodiment of the present invention configured as illustrated by
In the recuperated Brayton cycle, after passing through the heat recovery system, the heated working fluid expands through a turbine, and is subsequently communicated to a 4-stream heat exchanger 518 that regasifies the liquefied natural gas in multiple stages, and simultaneously works as a recuperator to pre-heat the high-pressure working fluid exiting from the compressor 516. Since the nitrogen is pre-heated, lower temperatures are obtained at the compressor outlet, and therefore the compressor operates at lower pressure ratios in comparison to a non-recuperated Brayton cycle. Thus, higher electrical efficiencies may be achieved for recuperated Brayton cycles as compared to non-recuperated embodiments.
As discussed herein, many variations of the present invention are possible. For example, a variety of variations of the embodiment of the present invention illustrated by the system 100 of
Furthermore, it is appreciated that while various embodiments are illustrated herein with nitrogen as a working fluid for the bottoming Brayton cycle, working fluids other than nitrogen may also be used. As noted, any suitable working fluid may be employed in the practice of the present invention. Typically, the working fluid is either inert or non-reactive with respect to the power plant environment. Suitable working fluids include, for example, argon, helium, carbon dioxide, and mixtures thereof. Depending upon the specific working fluid used, the various temperature and pressure ranges may vary accordingly, as will occur readily to those skilled in the art and in possession of this disclosure.
Embodiments of the present invention provide a number of advantages over known embodiments. For example, by pumping the LNG at two different pressure levels it is possible to have a very low associated increase of LNG temperature in the first compression stage. Further, the minimum useful temperature of the working fluid is decreased. Furthermore, the electrical efficiency of the bottoming cycle in comparison to a configuration regasifying LNG at one pressure level is significantly increased. In various embodiments, the flexibility of the system to fulfill the regasified LNG requirements for delivery/storage is increased, since very high LNG vaporization pressures can be achieved. Furthermore, pumping can be performed using a single pump with multiple pressure stages. Advantageously, the various embodiments disclosed herein can be easily retrofitted into existing power plants. The specific components of existing power plants can be suitably modified or replaced to provide power plants consistent with the various embodiments described herein. Further, the conversion of the LNG from its liquid state to a gaseous state can be achieved with the same or greater reliability as in simple cascaded configurations, since in some embodiments no additional equipment may be required. Finally, the volume of three stream heat exchanger may increase in comparison with a comparable two stream heat exchanger, and therefore a higher specific power per unit of volume may result. Lower CO2 emissions per unit of electricity generated per unit of fuel consumed may achieved, since a higher electrical efficiency and a higher power output (relative to comparable known systems) may be achieved using embodiments of the present invention.
Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item, and the terms “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation. If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., ranges of “up to about 25 wt. %, or, more specifically, about 5 wt. % to about 20 wt. %,” is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt. % to about 25 wt. %,” etc.). As a further example, the temperature denoted by the expression “between about −130° C. and about −100° C.” should be interpreted to include each the named temperatures −130° C. and −100° C. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity).
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
