LNG Receiving Terminal Process
Upon the arrival of an LNG transport ship, LNG is transferred into storage tanks through ship unloading pumps, liquid phase ship arms, and unloading pipelines. The evaporated gas (BOG) generated during unloading is partially returned to the cargo tanks of the LNG ship to balance the pressure inside the tanks. Another portion of BOG is compressed by BOG compressors and then condensed in a recondenser. The condensed BOG, along with the outgoing LNG, is pumped by high-pressure discharge pumps into the vaporizer for regasification.
The vaporizer converts LNG into gaseous natural gas. The natural gas is then pressure-regulated and metered before being sent into the transmission pipeline network. Additionally, it is also possible to directly compress the BOG to the outgoing pipeline pressure using booster compressors, bypassing the regasification process.
The LNG regasification/transmission system comprises submerged liquid pumps inside the LNG storage tank, a recondenser, high/low-pressure discharge pumps located outside the tank, vaporizer, and metering facilities.
Under normal operating conditions, only the Open Rack Vaporizer (ORV) / Integrated Full Containment Vaporizer (IFV) is operated. However, during maintenance or emergency peak shaving, the Submerged Combustion Vaporizer (SCV) can be operated in parallel.
Classification of LNG Vaporizers
Vaporizers are crucial equipment in LNG receiving terminals, and their structural designs vary based on the heat source they utilize.
1. Based on the utilization rate, vaporizers can be categorized into base-load vaporizers and peak-shaving vaporizers.
2. Based on the type of heat source, vaporizers can be classified as ambient vaporizers (using sources such as atmospheric air, seawater, or geothermal water), process vaporizers (using heat from thermal or chemical processes), and direct-fired vaporizers (using heat generated by fuel combustion).
Common types of LNG Vaporizers Found in Receiving Terminals
Air Ambient Vaporizer (AAV)
Intermediate Fluid Vaporizer (IFV)
Open Rack Vaporizer (ORV)
Submerged Combustion Vaporizer (SCV)
(1) Air Ambient Vaporizer (AAV)
The Air Ambient Vaporizer utilizes atmospheric air as the heat source to vaporize LNG. AAV features a simple structure and low operating costs. It can independently use ambient air as a heat source, completely avoiding emissions of pollutants and noise. Additionally, it can collect condensed water and melted ice water for production or domestic use.
However, AAV has some drawbacks. For instance, in low ambient temperatures, an additional heater is required to supplement the heat. Regular defrosting is also necessary to prevent icing on the surface of the vaporizer's pipes.
Due to the relatively low energy input from air heating, AAV is only suitable for systems with smaller installation scales and lower LNG vaporization requirements.
(2) Intermediate Fluid Vaporizer (IFV)
The IFV utilizes an intermediate heat transfer fluid to mitigate the effects of icing. Common intermediate fluids used include propane, isobutane, Freon, and ammonia.
In practical applications, this vaporizer operates in two stages. The first stage involves heat exchange between LNG and the intermediate fluid, while the second stage involves heat exchange between the intermediate fluid and the heat source fluid.
IFV occupies a small footprint and can provide stable vaporization rates. Moreover, there is no risk of seawater freezing. Its significant advantage lies in the comprehensive utilization of energy, specifically for cogeneration (combined heat and power) purposes.
This type of vaporizer has been widely adopted in base-load LNG vaporization systems, with significant usage in Japanese receiving terminals.
(3) Open Rack Vaporizer (ORV)
ORV uses seawater as the heat source and offers design simplicity, convenient operation, and easy maintenance. It is the mainstream type of vaporizer used in many LNG receiving terminals worldwide.
The mechanical structure of an LNG ORV is straightforward, with main external interfaces including LNG inlet, vaporized natural gas (NG) outlet, and seawater inlet/outlet. The heat exchange tubes are installed within a framework structure.
The fundamental unit of the vaporizer is the heat transfer tube, with multiple tubes arranged in a plate-like configuration. Each tube is welded to a gas header or liquid header to form a tube bundle plate, and several tube bundle plates form the vaporizer.
LNG enters from the lower main pipe and is distributed into individual small heat exchange tubes, flowing upwards within the tube bundle for heat exchange.At the top of the vaporizer, a seawater distribution device is installed. Seawater enters from the top and is distributed as a thin film along the outer wall of the tube bundle, transferring heat to the liquefied natural gas inside the tubes, heating it and causing vaporization.ORV requires minimal instrumentation and is easy to maintain. It operates without an open flame, ensuring high safety standards.
Additionally, to address external icing issues, there is a variation called SuperORV. It employs double-layered heat transfer tubes where LNG enters the inner tube through a bottom distributor, followed by gradual vaporization within the annular gap between the inner and outer tubes.
(4) Submerged Combustion Vaporizer (SCV)
SCV mainly consists of a water bath, burner, blower, flue gas injection pipe, enclosure, heat exchange tube bundle, and chimney.The fuel gas is burned inside the burner, and the high-temperature flue gas is discharged into the water bath through the lower exhaust pipe, causing turbulent motion in the water bath.
The LNG inside the heat exchange tubes undergoes sufficient heat exchange with the highly agitated water, resulting in heating and vaporization. Due to the direct contact and intense heat transfer between the high-speed flue gas and water bath, the heat transfer coefficient outside the tubes is high, ensuring uniform water bath temperature.
SCV offers rapid and convenient
Comparison of LNG Vaporizers
Currently, LNG receiving terminals commonly use ORV, IFV, SCV, and AAV. AAV has more restrictions and is relatively less utilized in receiving terminals.
The Open Rack Vaporizer (ORV) uses seawater as a heat medium and is more cost-effective compared to the Submerged Combustion Vaporizer (SCV).
However, it is important to consider that ORV incurs higher initial equipment investment costs, including seawater intake and discharge outlets, seawater pipelines, seawater pumps, and seawater purification equipment.
For base-load LNG receiving terminals, ORV should be the primary choice. However, ORV has limitations in cases of excessively low seawater temperatures, seawater containing harmful substances to the equipment, or when considering marine environmental protection.
SCV requires relatively lower initial investment, occupies a smaller footprint, and allows for quick start-up and shutdown. However, SCV requires fuel, resulting in higher operating costs compared to ORV.
The Immersed Flame Vaporizer (IFV) utilizes titanium tubes for heat exchange, enabling safe and stable operation even in the presence of poor seawater quality. The main challenge with IFV is the significant limitation in the selection of intermediate fluids.
Selection of Vaporizers
The selection of vaporizers should take into account their processing capacity, applicability, safety and reliability, flexibility, investment costs, usage conditions (base load, peak shaving, emergency use), environmental factors, and climate conditions. Depending on specific requirements, suitable vaporizers can be chosen individually or in combination for application.
1. Processing Capacity:
The processing capacity of a vaporizer should match the designed throughput of the receiving terminal. If the terminal only requires "liquid in, liquid out" with natural gas solely for on-site consumption, or if the annual processing volume is small and there is ample space available, Ambient Air Vaporizers (AAV) can be considered.
2. Adaptability and Reliability:
Considering the "functional positioning" of the receiving terminal, whether it is for base load, peak shaving, or a combination of both, the adaptability and reliability of the vaporizer become crucial. If continuous and reliable operation is required, the selection of vaporizers should include those suitable for handling the base load as well as for emergency peak shaving, such as Submerged Combustion Vaporizers (SCV) that allow for rapid start-up and shutdown.
3. Environmental Considerations:
The environmental conditions surrounding the receiving terminal primarily refer to external temperatures (including atmospheric and seawater temperatures) and the nature and parameters of the seawater. For example, when selecting Open Rack Vaporizers (ORV), factors such as the diameter and concentration of solid particles in seawater, the presence of heavy metal ions, the pH value, and other chemical properties of seawater must be considered.
Economic Considerations
The investment cost of vaporizers constitutes a significant portion of the overall investment in a receiving terminal. When selecting vaporizers, a comprehensive comparison should be made between their fixed investment and operating costs.
The Open Rack Vaporizer (ORV) uses a large amount of seawater and has certain quality requirements for seawater. It has higher investment and installation costs but lower operating costs.
The initial investment includes costs for the vaporizer equipment, supporting seawater intake and discharge outlets, seawater pipelines, seawater pumps, and seawater purification equipment. Operating costs should also consider the interval and expenses for reapplying corrosion protection coatings to the heat transfer surfaces.
Compared to the Submerged Combustion Vaporizer (SCV), ORV utilizes seawater, and the operational consumption mainly consists of the electricity consumption of seawater pumps. Therefore, its advantage lies in significantly lower operating costs, with the operating cost ratio between the two types being approximately 1:10.
SCV excels in terms of overall investment and installation costs, compact size, and operational flexibility. However, the fatal drawback of SCV is its high operating costs.
Under favorable seawater environmental conditions, using ORV is evidently the most reliable and cost-effective option.
However, if the seawater quality has a severe negative impact on the materials used in ORV (e.g., high concentrations of large suspended solids in seawater, which can significantly affect the corrosion protection coating on the heat transfer surfaces and shorten their service life), ORV should not be chosen.
Conclusion
Each type of gasifier has its own advantages and disadvantages, as well as specific operating environments that are suitable for them. To handle various conditions in LNG receiving terminals, it is a good choice to select 1-2 types of gasifiers for combination, which can leverage their respective strengths and compensate for inherent limitations.
Typically, when configuring gasifiers, a combination of 1-2 types is usually required. Currently, the ORV+SCV configuration is preferred when selecting gasifiers.
The open-rack vaporizer (ORV) is suitable for receiving terminals with large processing capacities and low operating costs. The submerged combustion vaporizer (SCV) has relatively higher operating costs but lower initial investment and reliable operation.
In cases where the seawater contains a high level of sediment or does not meet the required chemical properties, the intermediate fluid vaporizer (IFV) can be considered as an alternative.
Currently, there are 22 LNG receiving terminals in operation in China, with 13 more along coastline underway. The construction of LNG receiving terminals will greatly promote the importation of LNG resources in our country.
Gasifiers are an essential component of LNG receiving terminals, and the correct selection of gasifiers directly affects the safety, reliability, and economic aspects of terminal operations.
