NO20093562A1 - Gas supply system for alternating fuel or gas engines and boiling gas recondensation - Google Patents

Abstract

Gas supply system for AC or gas engines and BOG recondensation, wherein the BOG recondensation plant includes a cryogenic heat exchanger (1), a BOG compressor (2) with a BOG preheater (3), a nitrogen loop with a compander (4), gas fed to the engines is in the form of LNG from at least one loading tank (5) or condensate from the recondensation plant, and LNG or condensate is passed through an LNG evaporator (6) arranged in a closed loop, where the closed loop has at least one pump (7). ) and a medium heat source (8) circulating in the closed loop to provide a cold performance recovery during the production of gas fed to the engines. In addition, according to the present invention, an LNG optimizer (9) is arranged in front of the LNG evaporator, which allows heat exchange by means of available heat and cold performance in the BOG recondensation plant, and LNG or condensate respectively are fed as gas into the gas. motors.

Description

A system for alternative fuel or gas engines and coking gas recondensation

The present invention relates to energy integration and utilization of waste heat available in combined systems for fuel gas supply and boil-off gas (BOG, Boil-Off-Gas) recondensation, and in particular in a gas supply system for engines such as alternative fuel or gas engines for vessels that transport liquefied natural gas (LNG) and includes a BOG recondensation plant. However, this does not rule out the present system being installed on other vessels that use liquefied gas such as LNG, LPG etc. as fuel.

LNG vessels have traditionally had steam turbine propulsion systems, but are now changing to low-speed diesel, alternative fuel or gas propulsion systems.

The account below is based on a system for gas supply integrated with a BOG recondensation plant corresponding to NO patent application no. 20082158. The new main further developments are as follows: - Utilization of cold performance in LNG. Before the fuel is sent to the engine, LNG must be heated to ambient temperature. The available cold performance is utilized to cool down and condense BOG in an LNG optimizer. - Utilization of compression heat and waste heat from the compressor to heat up LNG or high-pressure LNG, e.g. compressor cooling water or hot nitrogen.

The previously combined system for BOG recondensation and gas supply to alternative fuel or gas engines is schematically shown in Figure 1. A process flow diagram illustrates the applicant's HGD 3rd generation recondensation system (Mark III) with a Gas Supply System for alternative fuel/gas engines. The two systems for recondensing BOG and gas supply with HP (high pressure) pumping and evaporation are two "separate" assemblies. In an LNG evaporator, cold performance is removed from LNG during evaporation and watering of this. The cold performance is removed using an external heat source and is not utilized.

Condensate from the BOG recondensation plant or LNG from cargo tanks supplied with the cargo pumps is sent to at least one high-pressure pump (HP) for feeding to the evaporator. LNG normally above the supercritical pressure is heated to a "gas" in a heat exchanger. The heat exchanger is referred to as an evaporator (evaporator, vaporizer). The high-pressure gas is then delivered to propulsion engines or gas turbines, not shown.

The outlet pressure for such a high-pressure pump is typically up to 30 MPa (300 bar) in the design case. The LNG temperature at higher pressure is typically in the range from -140 to -150°C. As LNG at high pressure is supercritical, it is not possible to distinguish between liquid and gas phase. Nevertheless, the watering of LNG to ambient temperature at high pressure is indicated as evaporation in this document.

The system shown in Figure 1 is based on the vaporization of LNG at high pressure with a heat source. To avoid using cooling water or steam from the engine room directly in a heat exchanger with LNG, i.e. the evaporator, a closed loop with an intermediate medium is used to heat the LNG. The intermediate medium can be a brine (brine), glycol mixture, hydrocarbon mixture or a cooling medium, hereinafter referred to as "brine".

To vaporize LNG, the closed loop includes an LNG vaporizer, a brine pump (brine pump), and a heat source in the form of a steam/hot water heater. In this way, the danger of getting LNG into the engine room in the event of internal leakage in the evaporator is prevented. Another important reason for using an intermediate loop is that the intermediate loop is safer with regard to the possibility of freezing.

The evaporator and its closed loop can be installed in a Frakt Kompressor Room (CCR-Cargo Compressor Room) together with the BOG recondensation plant, in a separate location or in the engine room.

LNG at -160 °C and 2 to 4 bar is pressurized using at least one cryogenic pump (HP pump) arranged in front of the LNG evaporator to typically 300 bar. The LNG is then evaporated and heated to ambient conditions - typically +40 °C in a heat exchanger, i.e. the evaporator.

In the evaporator, LNG is heated with brine, while the brine is watered with hot water from the engine (cooling water), steam, e.g. engine room, cooling water from process equipment in the "steam/hot water heater" mentioned above.

After the LN evaporator, the intermediate stream (brine) is pumped to a heat exchanger to heat the brine again. This heat exchanger, where the brine is heated, the heat source is indicated as the "steam/hot water heater".

Both systems for coke gas recondensation and fuel gas require energy to carry out their tasks. For cooling and condensing the decoction gas, a system to produce cold performance is necessary. The cold performance is produced with a nitrogen loop driven by a compressor cooled with cooling water. The LNG fuel gas supply needs heat to heat the LNG before feeding it to the engine. This means that one process, i.e. the recondensation system, needs cold performance, and the other process, i.e. the fuel gas supply system, needs hot performance. In order to achieve an overall reduction in energy and power consumption, the two process systems are integrated and optimized. There is thus an obvious need for reduced energy consumption for such systems.

According to the present invention, a system for gas supply to alternative fuel or gas engines and BOG recondensation is proposed, where the BOG recondensation plant includes a cryogenic heat exchanger, a BOG compressor which has a BOG preheater, and a nitrogen loop with a compander, the engines are fed with gas in the form of LNG from at least one cargo tank and/or condensate from the recondensation plant, and the LNG and/or condensate is passed through an LNG evaporator arranged in a closed loop having at least one pump and a heat source for an intermediate medium circulating in the closed loop to provide cold performance extraction when it produces gas fed into the engines, where an LNG optimizer is arranged in front of the LNG vaporizer, which allows heat exchange using available hot and cold performance in the BOG recondensation plant, and respectively LNG or condensate is fed as gas into the engines.

Preferably, respectively, LNG or condensate from the at least one cargo tank and the cryogenic heat exchanger can be directed into the LNG optimizer directly or via a separator, or BOG from the recondensation plant can be directed into the LNG optimizer to be fully or partially condensed in heat exchange with LNG and then fed into the separator for mixing, or alternatively into the cryogenic heat exchanger.

It is also possible to direct liquid from the separator via a suction tank (suction drum) into the LNG optimizer or return it to at least one loading tank.

Alternatively, hot nitrogen from the compressor stage in the compander can be directed into the LNG optimizer, or hot nitrogen can be mixed with nitrogen from an aftercooler connected to the compander, or hot nitrogen taken out upstream of at least one intercooler and/or the aftercooler connected to the compander can be directed to at least a separate heat exchanger arranged in a closed loop connected to the LNG vaporizer or at least one separate heat exchanger between the LNG optimizer and LNG vaporizer, or hot nitrogen taken out upstream of the cryogenic heat exchanger can be directed into the LNG optimizer.

Thus, a reduction of the total energy consumption is achieved:

- Utilization of cold performance in LNG when LNG is heated before feeding to the engine. The cold output is used for cooling and condensing coking gas. - Heating of LNG before feeding to the engine by using available "hot" output as cooling water from the compressors in the recondensation system such as BOG compressors and N2 companders, or other available cooling water or hot nitrogen.

Both concepts can be installed together in a system, or just one of them. The final configuration depends on the final concept and how the system will be operated after installation.

The present invention will now be described with reference to the accompanying drawings, in which: Figure 1 schematically shows a system for gas supply to alternative fuel or gas engines and BOG recondensation according to NO patent application no. 20082158; and Figures 2 to 6 schematically show various preferred embodiments of the present invention.

First, it is considered useful to provide a more detailed summary of the system for gas supply to alternative fuel or gas engines and BOG recondensation with respect to the above patent application. Both condensate from the BOG recondensation plant and LNG from at least one cargo tank 5 provided with a cargo pump 51 via a line 52 are sent to at least one HP pump 53 for feeding to an evaporator 6 to produce and deliver gas via a line 54 to the propulsion engines or gas turbines, not shown. Condensate can be returned to the cargo tank via a line 55. It is also possible to mix condensate and LNG in a separator 10 arranged upstream of the HP pump 53, where the separator is ventilated by means of a valve 101.

The system shown in Figure 1 is based on a vaporization of LNG at high pressure with a heat source. The evaporation is carried out using the closed loop including the LNG vaporizer (evaporator, vaporizer) 6, a brine pump 7, and a heat source in the form of a steam/hot water heater 8. LNG at typically -160 °C and 2 to 40 bar is pressurized using of at least one cryogenic pump, i.e. the HP pump 53, arranged in front of the LNG evaporator 6 to typically 300 bar. In the evaporator, the LNG is heated with the brine, while the brine is heated with hot water from the engine (cooling water), steam, e.g. engine room, cooling water from process equipment in the "steam/hot water heater" mentioned above.

The main components of the BOG recondensation plant are a cryogenic heat exchanger 1, a refrigerant compander 4, and a BOG compressor 2 which has a BOG preheater 3. As illustrated, the cryogenic heat exchanger is fed with cold output originating from compressor stages 41,42,43 in the compander also equipped with an expander as well as the BOG preheater. The compander is arranged in a loop including a nitrogen reservoir 42, at least one intercooler (intercooler) 13, an aftercooler (aftercooler) 12, a pump limit valve 46, and an expander circulation line 47. Cooling water for the intercoolers and aftercoolers is supplied via line 48. In addition, the BOG compressor 2 has several compressor stages 21, 22, 23 and in front of the inlet is also equipped with a BOG pre-heater adapted to the transfer of cold performance into the cryogenic heat exchanger. Correspondingly with the compander, the BOG compressor is connected to intermediate and aftercoolers 25,26,27, which exchange heat with cooling water supplied via a line 24.

According to the present invention, an essential purpose is to utilize the cold performance available in LNG before entering the evaporator 6 and to introduce the cold performance into the BOG recondensation plant or directly into the BOG. Thus, cold performance is able to cool and condense BOG before mixing the BOG stream with LNG from the cargo tanks, condensate from the BOG recondensation system or a combination of LNG from the cargo tanks and condensate from the recondensation system, cf. figures 2, 3 and 4.

The cold performance is utilized in an "LNG optimizer" 9, i.e. a device in which BOG or BOG condensate and LNG exchange heat with each other. Depending on the conditions of the BOG and LNG upstream of the LNG optimizer, the BOG is partially or fully condensed downstream of the LGN optimizer. BOG cooled in the LNG optimizer is typically mixed with condensate from the cold box in the form of a heat exchanger 1 or/and LNG from the cargo tanks 5 downstream of the cold box, in a mixer, not shown, or in a separator 10. If the BOG is not completely condensed downstream of the LNG optimizer, the stream will be condensed further in the mixer/separator unit after mixing BOG with the condensate from the recondensation plant and/or LNG from the storage tanks, cf. figures 2 and 3. Non-condensable components can be removed from the top of the separator via a suitable valve 101. From the separator 10 liquid liquid is sent to the fuel gas supply system or alternatively partly returned to the cargo tanks 5 via the line 55.

The LNG optimizer 9 can be installed upstream of the high-pressure pump 53; cf. Figure 2, or downstream of the same; cf. figure 2, depending on available conditions for BOG and LNG power and limitations on equipment.

In Figure 4, the BOG stream is precooled and partially condensed in the LNG optimizer 9 before the stream is reintroduced into the cold box, i.e. the cryogenic heat exchanger 1. This is an alternative solution instead of sending the BOG stream directly to the separator 10. The high-pressure pump 53 can be installed upstream or downstream of the LNG optimizer. A suction tank 11 can also be included.

For a configuration with the LNG optimizer 9, the pressure head of an LP pump 56 is typically between 3 to 40 bar to optimize the heating and cooling curve in the heat exchangers. The outlet pressure in the HP pump 53 depends on the engine requirements, e.g. 300 bar.

Instead of using an external heat source for watering the high-pressure LNG to the required temperature after the LNG optimizer 9, available heat output from cooling water or hot nitrogen gas from the compressor stages 41, 42, 43 upstream of the intercoolers or aftercoolers 13, 12 can be used. The main task is to utilize compression heat from the compressor stages to heat the LNG to ambient conditions before feeding it to the engine. Typical solutions to heat high-pressure LNG to reduce external heat sources can be: 1. Hot nitrogen from compressor stages 41,42, 43 due to compression heat can be sent directly to LNG optimizer(s) or mixed with nitrogen from the after-cooler 23 before entering the optimizer. The ratio between a flow from the after-cooler and from the compressor stages depends on the required performance in the LNG optimizer. Depending on the nitrogen temperature, cooled nitrogen can be returned downstream of the nitrogen compander 4, into the cold box 1 or upstream of the nitrogen expander 44; cf. figure 5. 2. Hot nitrogen withdrawn upstream of the intercooler(s) 13 and/or aftercooler 12 can be sent to at least one separate heat exchanger, not shown, between the optimizer 9 and LNG vaporizer 6 or to at least one heat exchanger 14 installed in the closed salt- the lake loop; cf. figure 6. Cold LNG is heated with water nitrogen from compressor stages 41,42,43. Cooled nitrogen is returned back to the main nitrogen loop downstream of the compander 4 at least one intercooler 13 and/or aftercooler 12. If necessary, parts of the nitrogen are cooled in the mill cooler(s)/aftercooler to meet the required temperature at the inlet to the compressor stages. 3. First, the LNG optimizer 9 utilizes the cold performance. After the utilization of the available cold performance in LNG, LNG fuel is sent to the next heat exchanger. If ambient temperature has not been reached, LNG can be further heated in this heat exchanger, e.g. The LNG vaporizer 6. The closed brine loop is heated with an external heat source 8. To reduce the external energy consumption, compression heat is used to heat the brine in a "brine preheater" 14. The compression heat is taken from nitrogen that bypasses the intercooler(s) and/or aftercooler 12 The compression heat can be utilized from one or more of the compressor stages 41,42,43. The system that utilizes the heat of compression after the last compressor stage is shown in Figure 6. The heat of compression can also be used directly in at least one heat exchanger, not shown, between the "optimizer" 9 and the "LNG evaporator" 6. 4. The compressor in the form of BOG compressor 2 and nitrogen compander 4 is usually water-cooled. The heat of compression from such compressors can be utilized directly by using the cooling water as the external heat source to heat the LNG before feeding it to the engine. The heat can be used to heat LNG directly or to heat the brine in the brine loop. 5. In addition to the standard water or air intercooler(s) and aftercooler for compressors, not shown, the heat of compression can be removed in a heat exchanger in parallel or in series with the intercooler(s) and aftercooler, not shown. In an intermediate loop, such as the brine loop, an intermediate medium can circulate between the heat exchanger and the LNG evaporator(s). This means that compression heat is transformed into the fuel-gas system and used to heat LNG. At the same time, compression heat is removed.

If there is insufficient waste heat or if the compressors are not operating, an external redundant heat source is required to heat the LNG to ambient conditions. If additional heat is required, such is typically added to the external heat source ("steam/hot water heater") shown in the accompanying figures.

Claims (9)

1. System for gas supply to alternative fuel or gas engines and BOG recondensation, where the BOG recondensation plant includes a cryogenic heat exchanger (1), a BOG compressor (2) with a BOG preheater (3), a nitrogen loop with a compander (4), gas fed to the engines is in the form of LNG from at least one cargo tank (5) or condensate from the recondensation plant, and LNG or condensate is passed through an LNG evaporator (6) arranged in a closed loop, where the closed loop has at least one pump (7) and a heat source ( 8) for an intermediate medium circulating in the closed loop to supply a cold performance extraction during the production of gas fed to the engines, characterized in that an LNG optimizer (9) is arranged in front of the LNG evaporator, which allows heat exchange using available hot and cold performance in The BOG recondensation plant, and respectively LNG or condensate are fed as gas into the engines. 2. System according to claim 1, characterized in that respectively LNG or condensate from the at least one cargo tank (5) and the cryogenic heat exchanger (1) is directed into the LNG optimizer (9) via a separator (10). 3. System according to at least one preceding claim, characterized in that BOG from the recondensation plant is directed into the LNG optimizer (9) to be partially or completely condensed in heat exchange with LNG and then fed into the separator (10) for mixing. 4. System according to claim 1 or 2, characterized in that BOG from the recondensation plant is directed into the LNG optimizer (9) to be partially or completely condensed in heat exchange with LNG and then fed into the cryogenic heat exchanger (1). 5. System according to claim 4, characterized in that liquid liquid from the separator (1) is directed via a suction tank (11) into the LNG optimizer (9) or is returned to the at least one loading tank (5). 6. System according to at least one preceding claim, characterized in that hot nitrogen from compressor stages (41, 42, 43) in the compander (4) is directed into the LNG optimizer (9). 7. System according to claim 6, characterized in that hot nitrogen is mixed with nitrogen from an aftercooler (12) connected to the compander (4). 8. System according to claim 1 or 2, characterized in that hot nitrogen taken upstream from at least one intercooler (13) or the aftercooler (12) connected to the compander (4) is directed to at least one separate heat exchanger (14) arranged in the closed loop connected to the LNG evaporator (6), or at least one separate heat exchanger between the LNG optimizer (9) and the LNG vaporizer (6). 9. System according to claim 8, characterized in that hot nitrogen taken out upstream of the cryogenic heat exchanger (1) is directed to the LNG optimizer (9).