Hydrocarbon Processing - August 2021 - GP-11

SPECIAL FOCUS: SMALL-SCALE LNG
Standardized, modular, small-scale
LNG with low lifecycle cost
F. VAN HEERDEN, PolaireTech, Sasolburg, South Africa;
F. HANEY, DyCat Solutions, Calgary, Alberta, Canada; and G. SKINNER, Gasconsult, London, UK
Natural gas has developed into the
fossil fuel of choice for power generation,
industrial use and domestic heating.
It produces 30%-40% lower carbon
emissions than oil or coal, and zero sulfur
emissions. The capital cost of gas-fired
power plants is half that of coal-fired facilities.
For this reason, LNG is regarded
as a high-growth market and is expected
to become a low-carbon bridging fuel,
pending full deployment of renewables
and green or blue hydrogen-based fuels.
At present, the bulk of LNG is produced
at low cost in large baseload plants
with capacities of up to 7 MMtpy per
train and having significant LNG storage
facilities. After shipment, LNG is typically
stored in a receiving terminal, regasified
and used as fuel in power plants and gas
grids. In addition to bulk supplies, LNG
production plants and receiving terminals
can provide competitively priced LNG to
smaller markets within a reasonable distance
by offering break-bulk volumes.
New LNG markets, including bunker
fuel and transportation fuel applications,
are developing fast due to environmental
pressures around carbon dioxide (CO2
nitrous oxides (NOX
) and sulfur (SOX
),
)
emissions. These markets are more distributed
and often cannot be economically
served by break-bulk volumes from
existing LNG storage facilities. They are
candidates for LNG sales from smallscale
LNG liquefaction facilities operating
on pipeline gas, flared gas, coal seam
gas, biogas and small gas fields. However,
these small-scale LNG plants face
challenging economics given their small
scale. Special emphasis must be placed
on developing a competitive offering
with the lowest lifecycle cost.
Achieving the lowest lifecycle cost requires:
*
A process configuration that strikes
Fuel gas
a competitive balance between
CAPEX and OPEX
* A project development and
implementation approach that
benefits from a lean, standardized,
modular design; low-cost shop
fabrication; and reduced project
schedule.
This article describes an optimized
solution to this challenge developed by
the authors.
Process configuration. The selected
process uses the feed natural gas in an optimized
system of expanders to provide
the necessary cryogenic refrigeration. As
a result, no external refrigerants, such as
nitrogen or mixed hydrocarbons, are required,
with the following benefits:
* No requirement for refrigerant
handling, storage, blending or
transfer facilities, reducing the
equipment count and capital cost
* No refrigerant make-up costs
* Reduced plot area
* High-efficiency process with low
power demand
* Low power demand translates to
low operating cost and low-cost
compression equipment.
FIG. 1a shows the process flow scheme,
which is suitable for a small-scale LNG
plant with an output of up to 200,000 tpy.
The concept is based on two expander
circuits: a warmer circuit shown in red and
a lower-temperature circuit shown in blue.
The cold outlet streams from expanders
EXP1 and EXP2 are routed to the cold
box, HX1, to provide cryogenic cooling
and are then returned to the expanders
by the recycle compressor, CP1. The expanders
are configured as expander-compressors,
with the compressor sections in
series with the primary compressor, CP1.
A patented feature is that a partial liquefaction
takes place in the low-temperature
expander, EXP2. This very efficiently converts
latent heat into mechanical work and
reduces power demand by approximately
15%. It further reduces the size and cost of
the main cryogenic exchanger, HX1.
This process arrangement has been independently
assessed as having a specific
power demand 10% below that of single
HX 1
CP 1
EXP 1
Feed gas
EXP 2
FIG. 1. Basic flow scheme of a small-scale LNG plant.
Gas Processing & LNG | JULY/AUGUST 2021 11
Storage
LNG

Hydrocarbon Processing - August 2021

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