Patent Application
20090193780
The present invention relates to a liquefied natural gas (“LNG”) fuelled power generation system for a marine vessel. The present invention relates particularly though not exclusively to a low emission power generation system for a marine vessel.
Natural gas (“NG”) is routinely transported from one location to another location in its liquid state as LNG. Liquefaction of the natural gas makes it more economical to transport as LNG occupies only about 1/600th of the volume that the same amount of natural gas does in its gaseous state. Bulk transportation of LNG from one location to another is most commonly achieved using double-hulled ocean-going vessels with cryogenic storage capability referred to as “LNGCs”. LNG is typically stored in cryogenic storage tanks onboard the LNGC, the storage tanks being operated either at or slightly above atmospheric pressure. The majority of existing LNGCs have an LNG cargo storage capacity in the size range of 120,000 m3 to 150,000 m3, with some LNGCs having a storage capacity of up to 264,000 m3. The temperature within an LNG storage tank will remain constant if the pressure is kept constant and vice versa. This phenomenon is referred to in the art as “auto-refrigeration”. Therefore, whilst LNG storage tanks are heavily insulated to limit the amount of LNG that boils off or evaporates, if some of the boil off gas is not released from the tank, the pressure and temperature within the tank will continue to rise.
It is common for an LNGC to receive its cargo of LNG at an export terminal located in one country and then sail across an ocean to deliver its cargo to an import terminal located in another country. LNG is normally regasified before distribution to end users through a pipeline or other distribution network at a temperature and pressure that meets the delivery requirements of the end users. Upon arrival at the import terminal, the LNGC berths at a pier or jetty and offloads the LNG as a liquid to an onshore storage and regasification facility located at the import terminal. Regasification of the LNG is most commonly achieved by raising the temperature of the LNG above the LNG boiling point for a given pressure. The onshore regasification facility typically comprises a plurality of heat exchangers or vaporisers, pumps and compressors. Such onshore storage and regasification facilities are typically large and the costs associated with building and operating such facilities are significant.
In recent times, environmental standards at import terminals have changed requiring a reduction in the emissions of pollutants from marine vessels in general as well as a reduction in the emissions of pollutants during regasification of LNG to natural gas. Traditional marine diesel oil fired engine technology is also no longer able to meet desirable air quality emission limitations in most developed countries. The present invention was developed to provide a power generation system fuelled by vaporized natural gas with a view to meeting air quality objectives by reducing or controlling emissions of pollutants into the environment.
According to one aspect of the present invention there is provided a power generation system for a marine vessel including a gas-fired power generation unit arranged to receive unodorized natural gas that has been vaporized from LNG stored onboard the marine vessel as a source of fuel gas, the power generation system further comprising an emission reduction system for treating the exhaust gases from the gas-fired power generation unit.
In one form, a source of fuel gas for the gas-fired power generation unit is boil-off gas removed from an LNG storage tank onboard the marine vessel. When the marine vessel includes an onboard regasification facility for vaporizing LNG to produce natural gas, a source of fuel gas for the gas-fired power generation unit in another form of the present invention is a stream of vaporized LNG generated by the onboard regasification facility. Advantageously, the power generation system may share power between a propulsion plant for moving the marine vessel from one location to another location and an onboard regasification facility for vaporizing LNG to produce natural gas.
In one form the emission reduction system includes a catalyst for removing at least one pollutant from the exhaust gas generated in use by the gas-fired power generation unit, the pollutant selected from the group consisting of: (a) nitrogen oxide; (b) particulates less than 10 micrometers in diameter; (c) volatile organic compounds; (d) sulfur dioxide (SOx); and, (e) carbon monoxide (CO), alone or in combination. In one form, the emission reduction system removes at least 95% of the nitrogen oxide from the exhaust gas or at least 90% of the volatile organic compounds from the exhaust gas or removes 25 to 70% of the carbon monoxide from the exhaust gas or removes 20 to 70% of particulates less than 10 micrometers in diameter from the exhaust gas.
In one form, the emission reduction system comprises a catalytic converter. In another form, the emission reduction system comprises a catalytic converter and an absorption catalyst in series. To reduce NOx emissions in the exhaust gas, the gas-fired power generation unit may be fitted with a pre-mixer combustion system for regulating the air/fuel gas mixture fed to the gas-fired power generation unit such that the air/fuel gas mixture is lean.
When boil-off gas from an LNG storage tank is used as a source of fuel gas for the gas-fired power generation unit, boil-off gas which is excess to the fuel gas requirements of the power generation system may be diverted to an onshore gas delivery system. In another form, a boil off gas generator for vaporizing LNG stored onboard the marine vessel may be used to produce forced boil off gas as a supplementary source of fuel gas to the power generation system.
In one form of the present invention, the power generation system further comprises a heat recovery system for generating steam to drive a stream turbine to provide supplementary power to the power generation system. Advantageously, the heat recovery system may be used to provide supplementary heat for vaporizing LNG stored onboard the marine carrier to produce natural gas.
To prevent formation of hydrocarbon condensate, the unodorized natural gas is directed to flow through a fuel gas conditioning unit to heat and pressurize the fuel gas prior to introduction of the unodorized natural gas to the gas-fired power generation unit. The gas conditioning unit comprises a compressor for pressurizing the gas and a temperature regulator to increase the temperature of the fuel gas prior to compression and combustion. The temperature regulator may use electrical heating, steam heating, or heat exchange with a circulating intermediate fluid as a source of heat for adjusting the temperature of the unodorized natural gas.
In one form, the marine vessel includes an onboard regasification facility for vaporizing LNG to produce natural gas, and the onboard regasification facility comprises one or more air fin vaporizers for exchanging heat between forced or natural draft ambient air and a circulating intermediate fluid. The air fin vaporizers may advantageously be used for either heating the intermediate fluid which in turn is used to transfer heat to vaporize LNG during regasification operations, or for cooling the intermediate fluid after the intermediate fluid has been used to recover heat from the exhaust gas of the gas-fired power generation unit.
In one form of the present invention, the air supplied to the gas-fired power generation unit is subjected to pre-cooling using an intermediate fluid circulating through an onboard regasification facility.
The unodorized natural gas that has been vaporized from LNG stored onboard the marine vessel may be used as the sole or predominant source of fuel gas to the gas-fired power generation unit whenever LNG is stored onboard the marine vessel to keep emissions of SOx to a minimum.
Preferably, the gas-fired power generation unit is a gas turbine.
In order to facilitate a more detailed understanding of the nature of the invention several embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:
Particular embodiments of a power generation system for a marine vessel are now described in the context of the marine vessel being an LNG carrier fitted with both a propulsion plant and an onboard regasification facility. The present invention is equally applicable to LNG carriers which are not provided with an onboard regasification facility. It is to be further understood that the present invention is not limited to use onboard an LNG carrier, but is equally applicable to any marine vessel which carries LNG in an storage tank onboard the vessel, for example a cruise ship or a support or supply vessel. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. Like reference numerals refer to like parts in relation to all of the Figures. Not all pumps, valves and other control elements have been shown in the Figures in the interest of simplicity.
A first embodiment of the power generation system 10 of the present invention is now described with reference to
With reference to
During combustion of any fuel gas, pollutants are produced which report to the exhaust gas. The quantity and type of pollutant produced in the exhaust gas depends on such relevant factors as the efficiency of combustion, the degree of air compression, the air-to-fuel ratio, the inlet temperature of the compressed air and the fuel gas, the humidity of the inlet air, ignition timing, efficiency of combustion and the type of fuel gas supplied to the gas turbine. In order to keep emissions to a minimum, the power generation system 10 of the present invention uses unodorized natural gas that has been vaporized from LNG stored onboard the marine vessel 14 as the source of fuel gas to the gas-fired power generation equipment 20. The only time when an alternative source of fuel such as marine diesel oil is used as a source of fuel for the marine vessel is when the level of LNG stored onboard the marine vessel 14 is insufficient to operate the gas-fired power generation unit 20.
Prior to liquefaction, wellhead gas is subjected to initial separation to remove particulates, free water, and condensate before being subjected to gas conditioning to remove acid gases (including carbon dioxide and hydrogen sulfide). The gas is then subjected to drying to reduce the water content, typically to a level below 50 parts per million. The dry gas is then subjected to further processing to remove mercury and heavy hydrocarbons. The final stage of conditioning involves removal of the liquefied petroleum gas (LPG) prior to liquefaction. Consequently, use of the natural gas produced from vaporized LNG as a source of fuel to the gas turbine 20 of the power generation system 10 of the present invention, produces a lower level of emissions and pollutants than would be produced by burning oil or coal to fire the burners of a traditional prior art steam turbine driven LNG carrier. Natural gas which is generated during vaporization of LNG is “unodorized” in that sulfur compounds are removed from well head gas prior to liquefaction. In contrast, pipeline natural gas sourced from an onshore gas distribution facility includes a sulfur containing odorant which is deliberately added to gas intended for use by consumers prior to distribution for the purpose of facilitating detection of leaks. The use of “unodorized” vaporized LNG leads to a reduction in the level of sulfur dioxide produced in the exhaust gas from the gas turbine 20 of the present invention. LNG also does not contain heavy hydrocarbons (which have been removed during gas conditioning prior to liquefaction) and this leads to reduction in the particulates present in the exhaust gas produced by the gas turbine 20 of the present invention compared with a gas turbine operated using odorized natural gas from an onshore gas distribution facility.
LNG is stored onboard the LNG carrier 12 in one or more insulated cryogenic storage tanks 28. The storage tanks can be membrane tanks or Moss-style tanks or prismatic tanks. The insulation on the LNG storage tanks 28 allows some of the LNG to warm over time and return to its gaseous form (a process referred to in the art as “boil off”). The LNG storage tanks 28 are operated in such a manner as to maintain a fairly constant tank pressure by removal of the boil off gas. Removal of the boil off gas allows the remaining LNG to be maintained at a constant cold temperature in its liquid form. An average of approximately 0.15 percent of the LNG will boil off each day under normal operating conditions.
The unodorized natural gas is used as one of the sources of fuel for the gas turbines 20 of the present invention is derived from one or more of the following sources: a) natural boil off gas from the LNG storage tanks 28; b) forced boil off gas from the LNG storage tanks 28; and, c) LNG vaporized to natural gas using the onboard regasification facility 16. Each of these options is described in greater detail below.
In the embodiment illustrated in
With reference to the embodiment illustrated in
With reference to the embodiment illustrated in
In the embodiments illustrated in
In the embodiments illustrated in
The post-combustion emission reduction system 40 used for the system and process of the present invention is capable of removing one or more pollutants selected from the group consisting of: nitrogen oxides (NOx), particulates less than 10 micrometers in diameter (PM10), volatile organic compounds (VOC), sulfur dioxide (SOx), and carbon monoxide (CO) from the exhaust gas. To achieve target emission reduction control limits, the emission reduction system is configured to be capable of removing at least 95% of the nitrogen oxide, and/or at least 90% of the volatile organic compounds, 25-70% of the carbon monoxide and/or 20-70% of the particulates less than 10 micrometers in diameter (PM10) from the exhaust gas.
The emission reduction system 40 may rely on one or more of the following alone or in combination: a catalytic conversion process; a selective catalytic reduction (SCR) process; an oxidation catalyst; a NOx reduction catalyst; an absorption catalyst; or an adsorption catalyst. An oxidation catalyst is an absorption-type catalyst which is particularly suited to the removal of carbon monoxide emissions from the exhaust gases which are directed to flow over a catalyst which oxidizes the carbon monoxide present in the exhaust gas to produce carbon dioxide. SCR is particularly suited to the reduction of NOx emissions from an exhaust gas. SCR relies on the injection of a suitable dose of aqueous urea or ammonia (anhydrous ammonia or aqueous ammonium hydroxide) into the exhaust gas upstream of an catalyst to act as a reducing agent. The oxides of nitrogen present in the exhaust gas react with ammonia and oxygen on the surface of the catalyst to form nitrogen (N2) and water (H2O). Maximum catalyst performance is achieved when the temperature of the exhaust gas fed to the catalyst falls within a specific target range which is a function of the type of catalyst being used. An SCR system is best suited for use on an engine which is operated at essentially constant loads.
With reference to the embodiment illustrated in
In another embodiment illustrated in
To regulate the temperature of the exhaust gas fed to the post-combustion emission reduction system 40, the hot exhaust gas from the gas turbine 20 is cooled to a target inlet temperature range before the exhaust gas is allowed to enter the emission reduction system 40. With reference to the embodiment illustrated in
Downstream of the emission reduction system 40, the exhaust gas is subjected to further cooling through heat exchange with a heat transfer fluid, such as boiler feed water, using a waste heat boiler 48 to produce a stream of low pressure steam 46. In the first heat recovery unit 44 upstream of the absorption-type catalyst system 42, heat is transferred from the hot exhaust gas to the stream of low pressure steam 46 generated by the waste heat boiler 48. In the embodiment illustrated in
During its passage through the steam condenser 50, the high pressure steam from the first heat recovery unit 44 exchanges heat with a circulating intermediate fluid. The circulating intermediate fluid enters the steam condenser 50 as cool intermediate fluid and exits the steam condenser 50 as warm intermediate fluid. If the temperature of exhaust gas downstream of the waste heat boiler 48 is higher than the temperature of the warm intermediate fluid downstream of the steam condenser 50, additional heat can be recovered from the exhaust gas through heat exchange with the warm intermediate fluid, if desired, by passing the exhaust gas through a second heat recovery unit 52. Suitable intermediate fluids include formate, glycol, propane, sea water or fresh water or any other fluid with an acceptable heat capacity and boiling point that is commonly known to a person skilled in the art.
In the embodiment illustrated in
When the LNG carrier 12 is fitted with an onboard regasification facility 16 and the LNG carrier 12 is conducting regasification operations, the warm intermediate fluid which has passed through the steam condenser 50 and, optionally, the second heat recovery unit 52, can be circulated through the onboard regasification facility 16 to provide a source of heat for use in the vaporization of LNG to natural gas during regasification operations. The stream of cool intermediate fluid which is returned from the onboard regasification facility 16 is then recirculated through the steam condenser 50 and the second heat recovery unit 52 to recover heat from the exhaust gas in the manner described above.
It is to be understood that while the onboard regasification facility 16 can use a variety of other sources of heat for vaporization of LNG, the use of forced or natural draft ambient air as a primary source of heat for vaporization of LNG onboard the marine vessel 12 is preferred to keep emissions to a minimum compared with other regasification technologies that rely on the use of seawater or the burning of liquid fuels as the primary heat source for vaporization. Advantageously, when the onboard regasification facility 16 comprises one or more air fin vaporizers 60, the warm intermediate fluid can be circulated through the one or more air fin vaporizers for cooling of the intermediate fluid using forced or natural draft ambient air when the LNG carrier 12 is not performing regasification operations or is underway. In this way the air fin vaporizers serve as heat exchangers with essentially two independent functions—a) heating of LNG during regasification operations, and b) cooling of a circulating intermediate fluid at all other times.
Now that several embodiments of the invention have been described in detail, it will be apparent to persons skilled in the relevant art that numerous variations and modifications can be made without departing from the basic inventive concepts. By way of example, the air supplied to the gas turbine 20 air compressor 24 can be pre-cooled to improve performance, for example using the cooled circulating intermediate fluid returning from the onboard regasification facility 16. In an analogous manner, the fuel gas fed to the gas-fired power generation unit 20 can be pre-heated to improve efficiency and reduce emissions using heat recovered from the exhaust gas by the circulating intermediate fluid, heating using low pressure or high pressure steam, or electric heating using power from the first or second generator 26 or 56, respectively. All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.
All of the patents cited in this specification, are herein incorporated by reference. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the summary of the invention, the description and claims which follow, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.
This application is a continuation of PCT/AU2007/001322, filed Sep. 7, 2007, and titled “Power Generation System for a Marine Vessel,” which claims priority to U.S. Provisional Patent Application Ser. No. 60/843,395, filed on Sep. 11, 2006, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60843395 | Sep 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/AU2007/001322 | Sep 2007 | US |
| Child | 12399546 | US |
