lng vaporizer selection based on site ambient …€¦ · vaporizer (orv), 25% uses the submerged...
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LNG VAPORIZER SELECTION BASED ON SITE AMBIENT CONDITIONS
Dhirav Patel John Mak
Daniel Rivera Joanne Angtuaco
Fluor
ABSTRACT
There are numerous methods for regasification and the selection of an optimum process depends on plant site location, climatic conditions and the throughput capacities. Today’s the LNG landscape is changing. Many of these newer LNG import terminals are smaller in size and are mainly located in South East Asia and South America. These new terminals place a strong emphasize on energy efficiency and emissions. This paper highlights the results of a LNG vaporization screening study for LNG regasification facilities located in warm climate and cold climate regions of the world. The objective is to provide a guideline in the selection of an LNG vaporization design that is suitable for today’s terminals.
INTRODUCTION
Traditionally, base load regasification terminals have used two types of vaporizers: 70% uses the Open rack Vaporizer (ORV), 25% uses the Submerged Combustion Vaporizer (SCV) and the remaining 5% uses the Intermediate Fluid vaporizer (IFV). In addition to these vaporizers, other types of vaporizers such as the direct air vaporizers, the Ambient Air Vaporizers (AAV), have been used in smaller regasification plants and peak shaving facilities.
The vaporizer selection is project specific and is typically selected based on site conditions, environmental compliance, and energy efficiency. In the past, LNG regasification terminals were used to produce natural gas to supply sales gas pipelines and were considered utility companies. Integration with power plants and recovery of waste heat for heating were seldom practiced. With today’s high energy prices and concerns for carbon emissions, integration with waste heat from power plants can be attractive and may be justified on a case by case basis.
LNG Regasification terminals are built where there is shortage of gas supply. With the growth of shale gas in North America, many North America import terminals are unused and are being planned to be converted into export terminals. Similar shale gas growth is expected to occur in China, which will eventually slow down the amount of LNG import. Most of the new LNG terminals are expected to be built in the equatorial and subequatorial regions. These terminals will serve smaller markets, and the size of the terminals and regasification facilities will tend to be smaller.
Another development is the use of LNG regasification vessels and FSRU with built-in regasification facility. There are a number of projects in the planning and construction stage for LNG RV and FSRU vessels around the world (e.g. Indonesia, Lithuania and East Mediterranean), which testifies to their growing popularity. These regasification ships are constructed in ship yards and can be deployed in a shorter time than land-based plants.
FSRU can be used to supply fuel gas for power generation of a medium size power plant. Generally when used for power generation, the LNG sendout requirement is relatively low. For example, with an efficient combined cycle design, the regasification plant sendout requirement is about 100 MMscfd to support a 400 to 500 MW power generation station.
The countries where these new regasification terminals are located can be broadly classified into two regions. First, there is the equatorial countries where the site ambient temperatures are fairly constant and
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do not fall below 18°C. Second, there is the sub-equatorial region where the site ambient temperatures can fall below 18 °C during winter months.
The following countries fall under the equatorial region definition:
• Asian Countries (Southern India, Indonesia, Thailand, Malaysia, Singapore, Philippines)
• North American Countries (Mexico) • South American Countries (Brazil)
Whereas following countries may fall under the subequatorial definition:
• Asian Countries (China, Vietnam, Mid-West and Mid-East of India)
• South American Countries (Chile, Argentina) • European Countries (Spain, UK, France)
The paper presents the results of a vaporizer study for these two regions based on proven and conventional vaporizer equipment. The vaporizer selection for two baseload regasification plants is also presented. Currently, there are other proprietary and more advanced vaporization systems but they are excluded from this evaluation.
Types of Vaporizers
Typical types of vaporizers that have been used worldwide for LNG regasification are:
• Open Rack Vaporizers (ORV)
• Submerged Combustion Vaporizers (SCV)
• Ambient Air Vaporizers (AAV) • Intermediate Fluid Vaporizers (IFV)
Open rack vaporizers (ORV) and submerged combustion vaporizers (SCV) are the most common vaporization methods in existing regasification terminals, which have generally been located in the subequatorial region. Recent LNG receiving terminal activities have been shifting to the equatorial region where the weather is warmer, and the use of intermediate fluid vaporizers (IFV) is found to be attractive. Important factors that should be considered in the LNG vaporizer selection process are:
• Site conditions and plant location
• Availability and reliability of the heat source
• Customer demand fluctuation
• Emission permit limits
• Regulatory restrictions with respect to the use of seawater
• Vaporizer capacity and operating parameters
• Safety in design
• Operating flexibility and reliability
• Capital and the operating cost Seawater (SW) Heating
LNG receiving terminals are generally located close to the open sea for ease of access to LNG carriers. Seawater is generally available in large quantities at low cost as compared to other sources of heat, and is the preferred heat source. The opposition is mainly from the environmental sensitive regions for the concerns on the negative impacts on marine life due to the cold seawater discharge and the residual chemical contents.
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Open Rack Vaporizer (ORV)
An Open Rack Vaporizer (ORV) is a heat exchanger that uses seawater as the source of heat. ORVs are well proven technology and have been widely used in Japan, Korea and Europe LNG terminals. The preferred seawater temperature for ORV operation is above 5°C. .
ORV units are generally constructed of aluminum alloy for mechanical strength suitable to operate at the cryogenic temperature. The material has high thermal conductivity which is effective for heat transfer equipment. The tubes are arranged in panels, connected through the LNG inlet and the regasified product outlet piping manifolds and hung from a rack (Figure 1). The panels are coated externally with zinc alloy, providing corrosion protection against seawater. ORVs require regular maintenance to keep the finned tube surface clean.
The panel arrangement feature provides ease of access for maintenance. Depending on the design of the units, it is also possible to isolate sections of the panels and vary the load on the units. The unit can be turndown to accommodate fluctuations in gas demand, gas outlet temperature and seawater temperature.
For large regasification terminals where significant amounts of water are required, in-depth evaluation and assessment of the seawater system must be performed. Often, late design changes are very difficult and costly to implement, thereby, the key issues and design parameter must be established early in the project, such as:
• Is the seawater quality suitable for operating an ORV system?
• Does the seawater containing significant amounts of heavy metal ions? These ions will attack the zinc aluminum alloy coating and will shorten its life.
• Does the seawater contain significant amount of sand and suspended solids? Excessive sediment will cause jamming of the water trough and the tube panel. Proper seawater intake filtration system must be designed to prevent silts, sands and sea life from reaching the seawater pumps and exchangers.
• The design must consider the environmental impacts of the seawater intake and outfall system, and minimize the destruction of marine life during the construction period and normal plant operation.
• Chlorination of the seawater is necessary to slow down marine growth. However, residual chlorine in the seawater effluent can impact the marine life and the usage must be minimized.
• Seawater discharge temperature must comply with local regulation. The temperature drop of seawater is typically limited to 5°C in most locations.
• Location of the seawater intake and outfall must be studied to avoid cold seawater recirculation.
• If site is located in a cold climate region, supplementary heating may be necessary to maintain the outlet gas temperature. Boiloff gas from LNG storage tanks can be used as fuel to these heaters.
• Is a backup vaporization system provided? This may be necessary during partial shutdown of the seawater system or during peaking demand operation.
• Is the regasification facility located close to a waste heat source, such as a power plant? Heat integration using waste heat can reduce regasification duty and would minimize the environmental impacts.
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Natural Gas To Metering
Seawater To Outfall
LNG
Seawater in
SeawaterIntake Pumps
Figure 1: Open Rack Vaporizer Flow Scheme
Fuel Gas (FG) Heating
LNG vaporization using fuel gas for heating typically consumes approximately 1.5 % of the vaporized LNG as fuel, which reduces the plant output and the revenue of the terminal. Because of the high price of LNG, SCVs are sometimes used during winter months to supplement ORV, when seawater temperature cannot meet the regasification requirement. They can also be used to provide the flexibility in meeting peaking demands during cold seasons. The SCV burners can be designed to burn low pressure boil-off gas as well as letdown sendout gas.
Submerged Combustion Vaporizers (SCV)
A typical SCV system is shown in Figure 2. LNG flows through a stainless steel tube coil that is submerged in a water bath which is heated by direct contact with hot flue gases from a submerged gas burner. Flue gases are sparged into the water using a distributor located under the heat transfer tubes. The sparging action promotes turbulence resulting in a high heat transfer rate and a high thermal efficiency (over 98%). The turbulence also reduces deposits or scales that can build up on the heat transfer surface.
Since the water bath is always maintained at a constant temperature and has high thermal capacity, the system copes very well with sudden load changes and can be quickly started up and shutdown.
The bath water is acidic as the combustion gas products (CO2) are condensed in the water. Caustic chemical such as sodium carbonate and sodium bicarbonate can be added to the bath water to control the pH value and to protect the tubes against corrosion. The excess combustion water must be neutralized before being discharged to the open water.
To minimize the NOx emissions, low NOx burners can be used to meet the 40 ppm NOx limit. The NOx level can be further reduced by using a Selective Catalytic Reduction (SCR) system to meet the 5 ppm specification if more stringent emission requirements are needed, at a significant cost impact.
SCV units are proven equipment and are very reliable and have very good safety records. Leakage of gas can be quickly detected by hydrocarbon detectors which will result in a plant shutdown. There is no danger of explosion, due to the fact that the temperature of the water bath always stays below the ignition point of natural gas.
The controls for the submerged combustion vaporizers are more complex when compared to the open rack vaporizers (ORV). The SCV has more pieces of equipment, such as the air blow, sparging piping and the burner management system which must be maintained. SCVs are compact and do not require much plot area when compared to the other vaporizer options.
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Natural GasTo Metering
LNG
Air
Air Blower
Burner
Fuel Gas
Stack
Figure 2: Submerged Combustion Vaporizer Ambient Air Heating
Air is another source of "free" heat and would avoid the use of fuel gas and the generation of greenhouse gas from SCVs. In the environmental sensitive parts of the world, the use of sea water may not be allowed and could also be difficult to permit. In this case, the use of ambient air heat is the next best choice.
Ambient Air Vaporizers (AAV)
Direct ambient air vaporizers are used in cryogenic services, such as in air separation plants. They are vertical heat exchanger and are designed for icing on the tube side and require defrosting. Automatic switching valves are installed to allow automatic defrosting using timers. They have been used for peak shaving plants, and smaller terminals. When compared to other vaporizer options, they require more vaporizer units and more real estate.
A typical AAV design configuration is shown in Figures 3. AAV consists of direct contact, long, vertical heat exchange tubes that facilitate downward air draft. This is due to the warmer less dense air at the top being lighter than the cold denser air at the bottom. Ambient air vaporizers utilize air in a natural or forced draft vertical arrangement. Water condensation and melting ice can also be collected and used as a source of service/potable water.
To avoid dense ice buildup on the surface of the heat exchanger tubes, deicing or defrosting with a 4-8 hour cycle is typically required. Long operating cycles lead to dense ice on the exchanger tubes, requiring longer defrosting time. Defrosting requires the exchanger to be placed on a standby mode, and can be completed by natural draft convection or force draft air fans. The use of force draft fans can reduce the defrosting time but would require additional fan horsepower. The reduction in defrosting time is typically not significant as the heat transfer is limited by the ice layers which act as an insulator.
Fog around the vaporizer areas can pose a visibility problem, which is generated by condensation of the moist air outside. The extent of fog formation depends on many factors, such as the separation distances among units, wind conditions, relative humidity and ambient temperatures.
The performance of ambient air vaporizers depends on the LNG inlet and outlet conditions and more importantly the site conditions and environment factors, such ambient temperature, relative humidity, altitude, wind, solar radiation, and proximity to adjacent structures.
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Ambient air heater is advantageous in hot climate equatorial regions where ambient temperature is high all year round. In the cooler subequatorial areas, where winter temperature is lower, supplementary heating may be required to meet the sales gas temperature.
Ambient Air
Natural Gas to Metering
LNG
Cold, dried air and water
Figure 3: Typical Ambient Air Vaporizer
Intermediate Fluid Heating
This LNG vaporizing via intermediate fluid utilizes Heat Transfer Fluid (HTF) in a closed loop to transfer heat to vaporize LNG. Three types of Heat Transfer Fluids are typically utilized for LNG vaporization:
• Glycol-Water
• Hydrocarbon Based HTF (Propane, Butane or Mixed Refrigerant)
• Hot Water
Glycol-water Intermediate Fluid Vaporizer (IFV)
This system typically uses glycol-water as an intermediate heat transfer fluid. Ethylene glycol or propylene glycol or other low freezing heat transfer fluids are suitable for this application. Heat transfer for LNG vaporization occurs in a shell and tube exchanger. Warm glycol-water flows through the intermediate fluid vaporizers where it rejects heat to vaporize LNG.
A simplified process sketch of these various heating options is shown in Figure 4. The IFV is a conventional shell and tube exchanger which is also known as Shell and Tube Vaporizer (STV). These glycol-water IFVs are very compact exchangers (vertical shell and tube design) due to the high heat transfer coefficients and the large temperature approach.
Currently, glycol-water intermediate fluid LNG vaporizers account for a small fraction (around 5%) of total LNG regasification markets worldwide. Some of the operating plants utilize air heater and reverse cooling tower as the source of heat.
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There are several options to warm the glycol-water solution prior to recycling it back into the shell and tube LNG vaporizers, such as:
• Air heater
• Reverse cooling tower
• Seawater heater
• Waste heat recovery system or fired heater
Using air for heating will generate water condensate, especially in the equatorial regions. The water condensate is of rain water quality which can be collected and purified for in-plant water usage and/or export as fresh raw water. Conventional air fin type exchanger consists of fin tubes are not designed for ice buildup. With the use of an intermediate fluid such as glycol-water, the glycol temperature can be controlled at above water freezing temperature hence avoiding the icing problems.
Similarly, the reverse cooling tower design, which extracts ambient heat by direct contact with cooling water via sensible heat and water condensation, would require an intermediate fluid. The heat of the cooling water can be transferred to the intermediate fluid by a heat exchange coil.
Seawater may be also be used. However, the use of seawater is more prone to exchanger fouling, and the exchanger (plate and frame type) need to be cleaned periodically. The plate and frame exchangers are very compact and low cost. Typically, spare seawater exchangers are provided for this option.
Fired heater may be used at the costs of fuel expense. For environmental compliance related to CO and NOx emissions, a selective Catalytic Reduction System can be fitted into the flue gas duct of the fired heater.
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Air Heater
AirOR
Turbine Exhaust / Fuel Gas
AND/OR
Waste Heat Recovery Unit or Fired Heater
SeawaterOutfall
Glycol-WaterCirculation Pumps
SurgeDrum
Saturated Air
ReverseCoolingTower
OR
SeawaterIntake
Atmos
Natural GasTo Metering
IntermediateFluid Vaporizer
LNG formSendout
ORSeawater
Heater Plate & Frame
Exchanger
Figure 4: Glycol-water Intermediate Fluid Vaporizer Integration with Different Heat Sources
Intermediate Fluid (Hydrocarbon) in Rankine Cycle
This system uses propane, butane or other hydrocarbon refrigerant as an intermediate heat transfer fluid (HTF). The use of a hydrocarbon avoids the potential freezing problems encountered with seawater. This vaporizer arrangement allows the use of cold seawater as low as 1°C to minimize fuel consumption in the downstream trim heater.
LNG heating is achieved using two heat exchangers operating in series: a first evaporator exchanger that uses the latent heat of propane condensation to partially heat LNG, and a second heat exchanger using seawater to further heat the LNG to the final temperature. The second exchanger is also used to vaporize propane that is recycled to the first exchanger.
Since the heating by seawater only occurs in the second exchanger, it avoids direct contact with cryogenic LNG, and hence freezing of seawater can be avoided. For this reason, seawater close to freezing can be used in this configuration. The basic flow arrangement is illustrated in Figure 5.
Propane or butane can also be used as a working fluid for power production with the addition of a propane gas expander. More power can be produced using the vaporized LNG as the working fluid in a natural gas expander. In most facilities, the pipeline gas pressure is lower than the HP sendout pump discharge pressure, and there is opportunity for power production. For example, when LNG is pumped to 100 barg or
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higher, heated and then expanded to 30 barg pressure, a significant amount of power can be generated. The expanded gas is cooled which needs to be reheated with seawater to meet the pipeline temperature.
For a typical LNG terminal, the power generated by the Rankine cycle and gas expansion can be used to reduce or even eliminate power import. The power can be generated without any fuel input or emissions which are very attractive for most terminals.
LNGVaporizer
IFVCirculation Pump
Natural GasTo Metering
LNG
SeawaterIntake
SeawaterOutfall
NG Trim Heater
IFVVaporizer
HP IFV vapor
liquid
LP IFV IFV Expander
NG Expander
SeawaterHeater
Figure 5: IFV LNG Vaporizers in Rankine Cycle Heat Integration with Power Plant
Where the regasification facility is located close to a power plant, a hybrid type system using the waste heat from the power plant and SCVs for trim heating can increase the thermal efficiency and improve the economics of the regasification process.
The conceptual heat integration scheme is shown in Figure 6. The hot exhaust gases from the gas turbine in the power plant pass through a direct contact heating tower with the exhaust heat to increase the temperature of a closed hot water circuit. This hot water is then circulated and injected to the water bath of the SCVs transferring the heat to the LNG.
The returned chilled water from the SCVs can be recycled back to pick the heat in the heating tower or can also be used to lower the gas turbine inlet temperature. A lower gas turbine inlet temperature can significantly increase the power output from the turbine. Typically, for each degree centigrade drop in air temperature, power output can be increased by about 1%. Aero-derivative turbine is more sensitive to change in air temperatures.
Depending on the available waste heat from the power plant, the fuel gas consumption in the SCVs can be reduced or even eliminated. In addition to energy savings, there is also reduction in CO and NOx emissions from the facility.
This SCV hybrid configuration offers flexibility in operation. It can be operated as a standalone submerged combustion unit, or it can use the warm water from the power plant for LNG regasification, without the submerged burner operating.
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Direct Contact Heating Tower
Gas Turbine Generator
LNG
Fuel Gas
Natural Gas To Metering
Purge
Ambient Air
Air Chiller
Atmos
Hot Exhaust
Fuel Gas
IFV (Water)
SCV
Circulation Pump
Power Plant
Hot Water
Cold Water
Figure 6: SCV Power Plant Integration COMPARISON OF VAPORIZER OPTIONS
The optimum choice of an LNG vaporization system is determined by the terminal’s site selection, the environmental conditions, regulatory limitations and operability considerations. It has to comply with the LNG industry’s requirements for minimizing life cycle costs. The selection should be based on an economic analysis in maximizing the net present value while meeting the local emissions and effluent requirements.
The following table compares the six vaporizer options in term of their applications, operation and maintenance, utility and chemical requirements, environmental impacts and relative plot sizes.
The six options considered in this study are:
• Option 1 uses ORV as in existing regasification terminals
• Option 2 uses propane as the intermediate fluid with seawater as the heat source.
• Option 3 uses glycol water as the intermediate fluid with air as the heat source.
• Option 4 uses glycol water as the intermediate fluid with seawater as the heat source.
• Option 5 uses SCV using fuel gas and waste heat from cogeneration plant as depicted in Figure 6.
• Option 6 uses ambient air vaporizer (AAV).
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Qualitative Comparison
Table 1: LNG Vaporization Option Qualitative Comparison
Options 1 2 3 4 5 6
HEATING MEDIUM
Seawater (SW) Propane (C3) / Seawater (SW)
Glycol-water (GW) / Air
Glycol-water (GW) /
Seawater
Hot Water (HW) Fuel Gas (FG))
/Waste Heat (WH)
Air
FEATURE Direct LNG vaporization
using sea water
Indirect LNG vaporization by
condensing propane which is
heated by seawater
Indirect LNG vaporization by glycol which is
heated by air fin exchanger
Indirect LNG vaporization by glycol which is
heated by seawater
Indirect LNG vaporization by hot
water which is heated by waste heat and SCV
Direct LNG vaporization
using air
MAJOR APPLICATION
70% base load plants use ORV
Cold climate application and
avoid freezing of seawater
For warm climate
application. IFV makes up 5 % of base load
plants
Similar to Option 3 except
seawater is used as the source of heating
For energy conservation with use of waste heat.
SCV is used in 25% of base load
plants
For warm climate
application, peak shavers
and where real estate is not a
constraint.
OPERATION & MAINTENACE
Seawater pumps and
filtration system
Maintenance of vaporizers and
cleaning of exchangers
Similar to Option 1 with addition of a glycol loop and propane
system
Similar to Option 2 with a glycol loop and
use of air as the source of
heat
More Complex, requiring
coordination with power
plant.
More complex control. Need to balance waste
heat and fuel gas to SCVs. Require coordination with
power plant operation
Cyclic operation, requiring
adjustment of the defrosting
cycle according to ambient changes
UTILITIES REQUIRED
Seawater and electrical power
Seawater and electrical power
Electrical power only
Seawater and electrical power
Fuel gas and electrical power
Electrical power only
CHEMICALS Chlorination for seawater treatment.
Similar to Option 1 but lower chlorination
None Similar to option 1 but
lower chlorination
Neutralization required for pH
control and NOx reduction by SCR
None
EMISSION & EFFLUENTS
Impacts on marine life from cold seawater and residual
chloride content
Impacts on marine life from cold seawater and residual
chloride content
No significant impact on
environment except dense
fog
Impacts on marine life from cold seawater and residual
chloride content
Flue gas (NOx, CO2 ) emissions and acid water
condensate discharge
No significant impact on
environment except dense
fog
SAFETY Leakage of HC from ORV to
atmosphere at ground level
Leakage of HC to atmosphere at
ground level. Operating a
propane liquid system is
additional safety hazard
Leakage of HC to glycol system
which can be vented to safe
location via surge vessel
Leakage of HC to glycol
system which can be vented to safe location
via surge vessel
Leakage of HC to water system which can be vented to safe location via the SCV stack and surge vessel
Leakage of HC from AAV to
atmosphere at ground level
PLOT PLAN Medium Size Medium Size Large Size Medium Size Small Size Large Size
Rankings of Vaporizers
Warm ambient locations:
In warm ambient locations, for site locations in equatorial zone, where site ambient temperature stays above 18°C, the ambient air vaporizers or the air heated intermediate fluid type vaporizer units can provide the full LNG vaporization duty without trim heating. In addition, there is potential revenue to be gained by collecting and marketing the water condensate from the air.
The 6 options in Table 1 are ranked for their performance in terms of environmental impacts, system operability and maintenance requirement. The ranking system is based on a score of 1 to 6, with 1 being the most desirable and 6 the least desirable. These scores are summed and the one with the lowest score is
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considered the most desirable option. The rankings are divided into two regions. Table 2 is for vaporizers in the equatorial zones where ambient temperature is always greater than 18°C and Table 3 is for vaporizers that operate in the subequatorial zones where ambient is less than 18°C.
For the hot climate zone, the environmental score for air heating is the top two most desirable (option 3 and 6) followed by seawater options (1 and 4). Option 5 uses fuel gas for heating in the SCV generating emissions and hence the least desirable. The use of propane as an intermediate fluid (Option 2) requires an additional propane system which is not required in a warm climate region and is also ranked low in the rating.
For operability and maintainability, air heating (option 3 and 6) is the simplest to operate and maintain. Option 3 using an intermediate fluid with the air heater, which eliminates the cyclic defrosting operation required for AAV and is ranked the most desirable. For this reason, option 3, the use of glycol and air heating is considered the most desirable. However, the score is only marginally higher than the AAV option. The final selection depends on other factors, such as plot space requirement, capital and operating costs.
Table 2: Vaporizer Rankings for Ambient above 18°C
Option Vaporizer / Heat Transfer Fluid Environmental Operability Maintainability
Total
Rank
1 ORV (SW) 4 3 3 10 3rd
2 IFV (C3/SW) 5 5 5 15 5th
3 IFV (GW/Air) 2 1 1 4 1st
4 IFV (GW/SW) 3 4 4 11 4th
5 SCV (HW (FG) /WH) 6 6 6 18 6th
6 AAV (Air) 1 2 2 5 2nd
Cold ambient locations:
In cold ambient locations for site locations in sub equatorial zones, where site ambient temperature drops below 18°C, heating medium systems using seawater or air may not be able to meet the vaporization duty and pipeline gas temperature. When the site ambient temperature is below 18°C, external heating may be required for all options and supplemental heating integrated with SCV or FH must be provided during the winter months.
Option 5 which uses waste heat from the power plant is the most desirable in the environmental ranking. However, this option requires coordination with the power plant and is more complex in terms of operability and maintainability, it is the least desirable.
In the cold climate areas, ambient air temperature is typically more severe than seawater, and the air heated options are most likely to use significantly more fuel in the winter time. The rankings of air heating (option 3 and 6) are higher than the use of seawater options (option 1 and 4) and are less desirable because of the longer period when operating the SCV or FH is required, which increases the fuel consumption and emissions. Therefore, in cold climate operation, the use of seawater heating in combination with SCV ranks the most desirable.
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Table 3 - Vaporizer Rankings for Ambient below 18°C
Option Vaporizer / Heat Transfer Fluid Environmental Operability Maintainability
Total
Rank
1 ORV - SCV (SW - FG) 2 1 3 6 1st
2 IFV - FH (C3/SW - FG) 4 5 5 14 6th
3 IFV - FH (GW/Air - FG) 5 3 1 9 3rd
4 IFV - FH (GW/SW - FG) 3 2 4 9 2nd
5 SCV (HW (FG) / WH - FG) 1 6 6 13 5th
6 AAV - SCV (Air - FG) 6 4 2 12 4th
The environmental and operability criteria ratings for the above two tables are different mainly due to the fuel gas consumption when using SCV or FH for the two different climatic regions. For the maintenance criteria, the rating was left unchanged between the two site conditions as the only difference is the use of SCV or FH which is common for all options.
Option 5 in Warm ambient locations is ranked 1 (the most desirable) in the environmental category as it is the only option that can avoid fuel gas usage when the power plant operation is operating. However, the SCV must be provided to support the total regasification duty in case the power plant is down for maintenance.
NUMBER OF VAPORIZER AND CAPACITY FOR BASELOAD PLANTS
The number and capacity of vaporizers for the above 6 options are analyzed for the two regasification plant capacities: 3 MTA and 0.3 MTA. The 3 MTA plant is considered as the typical baseload plant in recent projects. The 0.3 MTA is the plant size that can be used to supply fuel gas to a 300 MW combined cycle power plant and is considered as a “fit for purpose” regasification plant.
Table 4 and Table 5 summarize the number of vaporizers and operating capacities for each of the 6 options for these two plant capacities.
The numbers of vaporizers are determined by the maximum size manufactured by the vaporizer vendors, the operating philosophy and sparing requirements. The design capacities of these vaporizers are:
Vaporizer Maximum LNG ton per hour ORV 300
FV / SCV 200 AAV 5
3 MTA plant Vaporizer Design
As shown in Table 4, for the 3 MTA baseload terminals where ambient temperature is always above 18°C, vaporizer configuration can be a combination of 2 x 50% ORV/IFV and 1 x 50% SCV on standby. The number of AAV can be as high as 28 trains with 5 units per train. Note that only about half of the number of AAV is used for heating while the remaining is on the defrosting mode at any one time.
Where ambient temperature drops below 18°C, the number of SCVs must be increased to three to accommodate the higher duty. Each vaporizer operates at 50% of the design capacity. Alternatively, 2 SCV can operate at full load with on one SCV on standby.
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Table 4: Vaporizer Design and Capacity for 3 MTA Regasification Plant
Vaporizer Option 1 2 3 4 5 6 1 2 3 4 5 6
Heating Medium Fluid (HTF) SW C3 / SW
GW / Air
GW / SW
HW (FG) / WH
Air (AAV) SW C3 /
SW GW / Air
GW / SW
HW (FG) / WH
Air (AAV)
Minimum Site Ambient Temperature Above 18°C Below 18°C Number of Vaporizers 2 28 2 28 Operating Capacity of Each Vaporizer, % 50 15 50 15 Number of SCV's 1 - 3 Operating Capacity of Each SCV, % 50 - 50
0.3 MTA Vaporizer Design
For the smaller 0.3 MTA plant, the combination of the vaporizers can be 2 x 100% for ORV/IFV operating as shown in Table 5. The number of AAV can be as high as 4 trains with 5 units per train, with half of the number of AAV used for heating while the remaining is on the defrosting mode at any one time. Where the minimum site ambient temperature falls below 18°C, the number of SCVs must be increased to 2, with one operating and one on standby mode.
Table 5: Vaporizer Design and Capacity for 0.3 MTA Regasification Plant
Vaporizer Option 1 2 3 4 5 6 1 2 3 4 5 6
Heating Medium Fluid (HTF) SW C3 / SW
GW / Air
GW / SW
HW (FG) / WH
Air (AAV) SW C3 /
SW GW / Air
GW / SW
HW (FG) / WH
Air (AAV)
Minimum Site Ambient Temperature Above 18°C Below 18°C Number of Vaporizers 2 4 2 4 Operating Capacity of Each Vaporizer, % 100 50 100 50 Number of SCV's/Fired heater - 2 Operating Capacity of Each SCV/Fire Heater, % - 100
Operating with Low Temperature Seawater
When seawater drops during winter, ORVs can continue to operate but at a reduced rate, as long as the freezing temperature of seawater (typically at -1.5°C) has not been reached. Using the propane as the intermediate fluid as shown in Figure 5, performance of the IFV can be maintained even when the seawater temperature drops to 5°C. The unit can continue to operate down to 1°C seawater temperature, but with a much reduced LNG throughput. The reduction in LNG throughput versus seawater temperature drop is almost linear as shown in Figure 7. The exit gas from the IFV exchanger can be trim heated using the standby fired heater or SCVs.
There are potential fuel savings by operating with a low seawater temperature in very cold climate regions. This application has to be evaluated with the increase in investment of the additional equipment and operation requirement.
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Figure 7: Impact of Seawater Temperature on LNG Throughput SUMMARY / CONCLUSIONS
For fuel savings and minimizing greenhouse gas emissions, use of “free heat” from ambient air or seawater is the most desirable. Fuel gas should only be used for trim heating during cold winter months, used as a backup heating or for peak operation. The vaporizer design option selection is different depending on the ambient conditions. For the equatorial regions where ambient temperatures are fairly mild and stay above 18°C, the use of ambient air for heating is the optimum choice. Air heating can be integrated with a heat transfer fluid using air fin exchanger, or using the standalone Ambient Air Vaporizers. For the subequatorial regions, fuel gas firing is required during winter. Seawater heating has an advantage over air heating as the seawater heater can operate for a longer period than air heater, which reduces fuel gas consumption in the trim heating. Considering today’s smaller regasification terminals, particularly the “fit-for-purpose” design for power plant, the selection of vaporizer options can be quite different than the existing larger plants.
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