Hydrocarbon Processing - December 2021 - GP-10

EXECUTIVE VIEWPOINT
PGUs in LNG facilities are typically
classified as operating in island mode.
This mode is coined as such because the
PGUs are not connected to an external
power grid-e.g., a local municipality. If
the electrical grid is unstable, then the
LNG train would be forced to trip and
shut down, resulting in a loss of revenue.
In such installations, maintaining a
FIG. 1. LNG production rates of various combined-cycle power plants using SGT-800C1
in combination with cogeneration. As the power generation facility increases in capacity,
so does the range of LNG production for a given design.
cate balance exists between the amount
of steam available for process heating
relative to the electrical load demand.
Logically, the LNG train starts up from
the upstream gas processing unit and
cascades down to the liquefaction and
storage facilities. This standard startup
sequence results in a higher ratio of
process heating load to electrical power
demand during the first steps of a startup,
compared to the ratio of design to
steady-state. Exceeding the design ratio
indicates an imbalance to the system that
would require supplemental process heat
to be supplied by another source.
Adding a furnace to provide the heat
necessary during startup is a potential solution.
However, this will result in higher
emissions and add a piece of equipment
typically not used in normal operations.
A temporary furnace rental is another
solution, but it may not be feasible, depending
on the LNG train capacity. In
some instances, it may also be possible to
address the heat imbalance by proposing
an alternate startup sequence.
The best option will ultimately be
FIG. 2. Refrigerant compressor driver
efficiency: Gas turbine (top) vs.
electric motor (bottom).
design stages of the LNG train in relation
to the PGU. At an eLNG plant with
a PGU operating in island mode, a delidictated
by the unique requirements of
the plant and operator, and it can be determined
only by evaluating the power
plant and liquefaction island together,
holistically.
Electrical grid stability. Equally important
to ensuring a reliable supply of
process heat is the stability of the plant's
electrical grid.
10 NOVEMBER/DECEMBER 2021 | GasProcessingNews.com
balance between power generation and
electrical load consumed is crucial. Active
and reactive power imbalance can
lead to frequency and voltage deviations,
and without sufficient reserve, frequency
and voltage regulation may present serious
problems. Therefore, the electrical
system must ensure voltage and frequency
stability in all scenarios at all times. A
comprehensive electrical system evaluation
is essential for ensuring reliability at
any continuous process facility; however,
it is particularly so when renewables
enter the energy mix.
Several LNG projects across the world
are now evaluating the use of renewable
power sources to reduce emissions. More
specifically, this is being considered in
combination with conventional natural
gas generation to create a hybrid system.
Integration of renewables adds benefits,
but it also complicates the stabilization
of the plant electrical grid. For
example, a photovoltaic (PV) plant can
provide support during over-frequency
cases. The challenge, then, is making the
energy system flexible enough to cope
with this transition.
The renewable power output from
solar or wind is highly dependent on renewable
source availability. A possible
solution to balance this intermittence is
to use energy storage. Power production
and demand can be balanced by storing
electricity in times with adequate wind
and sun, and then feeding this power into
the grid on windless and cloudy days.
Storage technologies are differentiated
according to the amount of energy
that can be stored and the length of time
it can be stored. Battery storage systems
(BSS) are examples of conventional
methods that store energy for short-term
periods (i.e., minutes or hours).
Pumped-storage power plants are employed,
or hydrogen can be used as an energy
vector when it comes to mass energy
storage for more extended periods. BSS
also help distributed generation operators
to integrate and better utilize power
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Hydrocarbon Processing - December 2021

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