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
1
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.
2
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.
3
Natural Gas
To Metering
Seawater in
LNG
Seawater
To Outfall
Seawater
Intake 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.
4
Natural Gas
To Metering
Stack
LNG
Air
Burner
Air Blower
Fuel Gas
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.
5
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.
6
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.
7
Natural Gas
To Metering
Waste Heat
Recovery Unit
or Fired Heater
Intermediate
Fluid Vaporizer
Atmos
AND/OR
Turbine Exhaust / Fuel Gas
Reverse
Cooling
Tower
Saturated
Air
OR
Air Heater
Air
LNG form
Sendout
OR
Surge
Drum
Seawater
Intake
OR
Seawater
Outfall
Seawater
Heater
Plate & Frame
Exchanger
Glycol-Water
Circulation Pumps
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
8
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.
HP IFV vapor
LP IFV
LNG
Vaporizer
IFV Expander
IFV
Vaporizer
NG Trim
Heater
Seawater
Intake
Seawater
Outfall
LNG
Natural Gas
To Metering
liquid
Seawater
Heater
IFV
Circulation Pump
NG Expander
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.
9
SCV
LNG
Natural Gas To
Metering
Hot Water
Atmos
Fuel Gas
Cold Water
Direct Contact
Heating Tower
Circulation
Pump
Hot Exhaust
Fuel Gas
Purge
Ambient Air
Air Chiller
Gas Turbine Generator
IFV (Water)
Power Plant
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).
10
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
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
Maintenance of
vaporizers and
cleaning of
exchangers
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
11
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.
12
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
ORV
FV / SCV
AAV
Maximum LNG ton per hour
300
200
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
Heating Medium Fluid (HTF)
1
SW
2
5
6
1
HW
C3 / GW / GW /
Air
(FG) /
SW
SW Air
SW
(AAV)
WH
Minimum Site Ambient Temperature
3
4
2
3
C3 /
SW
GW /
Air
Above 18°C
4
5
6
HW
GW /
Air
(FG) /
SW
(AAV)
WH
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
Heating Medium Fluid (HTF)
1
SW
2
5
6
1
HW
C3 / GW / GW /
Air
(FG) /
SW
SW Air
SW
(AAV)
WH
Minimum Site Ambient Temperature
Number of Vaporizers
Operating Capacity of Each Vaporizer, %
Number of SCV's/Fired heater
Operating Capacity of Each SCV/Fire
Heater, %
3
4
Above 18°C
2
3
C3 /
SW
GW /
Air
4
5
6
HW
GW /
Air
(FG) /
SW
(AAV)
WH
Below 18°C
2
4
2
4
100
50
100
50
-
2
-
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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LNG Throughput, ton/hr
250
200
150
100
50
0
0
1
2
3
4
5
6
Seawater Temperature, °C
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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