50
LNG
INDUSTRY
MARCH
2016
compressor. The working fluid can be nitrogen, methane or a
mixture of both. While methane performs better at
precooling and liquefaction stages, nitrogen yields better
efficiency at the subcooling stage, avoiding methane
operating at low pressure. In offshore operations, nitrogen is
preferred as the inert gas is a safer working medium.
The configuration of the expander cycle
2
has been
optimised based on a two-stage expander cycle, as shown in
Figure 3. With the two-stage expander design, the heat
composite curves of the feed gas and the refrigerant can be
closely matched, as shown in Figure 4. Adjusting the flow
and pressure to the inter-stage compressor optimises the
cold section of the process.
The gas expander cycle is robust as it can be operated
with a wide range of feed gas compositions. Operation
requires minimal operator attention, which is important for
remote and offshore operation. The gas phase system can be
started up and shutdown quickly and safely.
LNG production and relative
cost
LNG production rate is primarily dependent on the available
power to the refrigeration compressors. In a standalone SMR
design, the gas turbine driver output determines the LNG
production rate. With the addition of the ORC power, LNG
production can be increased for both the SMR and expander
processes.
Table 1 summarises the LNG production and the relative
cost for the four design options. The comparison is based on
a typical gas turbine driver with power to produce
1 million tpy of LNG using the standalone SMR process.
For the SMR process, with additional power from the
ORC plant, LNG production can be increased by 35% to
1.35 million tpy. The cost of the ORC plant only adds 10% to
the SMR standalone plant. The incremental cost of the ORC
can be justified with the higher production. When compared
to the gas turbine power plant, the ORC incremental power
cost is significantly lower, mainly due to the process
simplicity, and the use of the low cost carbon steel
equipment.
The LNG production rates with the expander cycle are
more sensitive to ambient temperatures, as less compressor
power is required for cold ambient operation. Rates range
from 0.8 million tpy to 1 million tpy, for an ambient
temperature range from 77°F to 50°F. With the addition of
ORC power, the expander cycle can produce 1 million tpy to
1.2 million tpy of LNG.
The standalone expander cycle plant cost is
approximately 10% higher than the standalone SMR plant
due to the addition of the two expander compressor sets,
and the larger cold box. On the other hand, the refrigeration
compression cost is lower due to the elimination of the
temperature control and two-phase operation. With the
addition of the ORC plant, the overall cost is approximately
20% higher than the standalone SMR design.
Note that the LNG plant cost is based on Inside Battery
Limits (ISBL) equipment only and does not include the cost
for utilities, offsites and storage.
LNG plant specific power
consumption
When power produced from the ORC plant is used to
operate the first stage of the refrigeration compressors,
power consumption by the liquefaction plant is reduced. The
specific power is based on a lean gas with a feed pressure
of 850 psig. The range of specific power consumption for
the four designs is summarised in Table 2. The wider range
for the SMR and expander cycle specific power is due to the
range of ambient temperature from 50°F to 77°F. The power
consumption is reduced per ton of LNG production in cold
climate operation.
Conclusion
The use of the ORC can reduce energy consumption and CO
2
emissions when compared to the standalone liquefaction
process in mid scale LNG plants. The ORC is a proven
process, but is yet to be applied to LNG liquefaction. The
application can provide benefits for both the SMR and the
gas expander cycle that is customised for mid scale LNG
liquefaction plants. In summary, the advantages of the
ORC power integration to LNG liquefaction for mid scale
liquefaction plants include the following:
Lower CAPEX and OPEX cost per unit of LNG liquefied.
Lower refrigerant power consumption per unit of LNG
liquefied, reducing overall carbon footprint.
Higher LNG throughput for a given
refrigeration compressor driver.
References
1. Fluor Patent Application Serial
Number 62125 J 808: ‘Methods
and Configuration for LNG
Liquefaction ORC-SMR Process.’
2. Fluor Patent Application Serial
Number 62/252247: ‘Methods
and Configuration for LNG
Liquefaction ORC-Twin Expander
Process.’
3. ‘Handbook of Natural Gas
Transmission and Processing’,
third edition, (2 March 2015),
Elsevier, Gulf Professional
Publishing.
4. ‘Handbook of Liquefied Natural
Gas,’ first edition, (2014), Elsevier,
Gulf Professional Publishing.
Table 1.
LNG production and relative costs
Liquefaction
process
Standalone SMR Integrated SMR
+ ORC
Standalone
expander
Integrated
expander + ORC
LNG production
(million tpy)
1.00
1.35
0.8 – 1.0
1.0 – 1.2
Relative capital
cost
100%
110%
110%
120%
Table 2.
LNG plant specific refrigeration power consumption
Liquefaction process SMR
SMR + ORC Expander
Expander +
ORC
Specific refrigeration
power (kW/t)
265 – 300
195 – 220
330 – 365
250 – 285
