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January 2020
efficient best available technologies in reducing its carbon
footprint throughout the natural gas value chain, including LNG
production.
LNG plants have traditionally been developed as integrated
projects by stranded natural gas asset owners controlling the
entire value chain. Focus was placed on reducing capital costs
due to its high cost outlay and long payback periods at these
remote sites. These projects often overlooked operating cost
savings, as typically feed gas cost was minimal for an integrated
project developer. Further, there were no strict regulations in
place to reduce carbon emissions in jurisdictions where LNG
projects were developed. Plant improvements were focused
around equipment design enhancements, such as improving gas
turbine metallurgy to withstand higher exhaust temperatures,
optimising compressor driver arrangement and increasing heat
transfer area per unit volume in heat exchangers. Although the
benefits of waste heat recovery and combined cycle power were
well understood, the vast majority of LNG plants continued to be
built with open cycle industrial gas turbines as an inefficient but
capital cost-friendly approach, with little or no consideration paid
to CO
2
emissions and operational efficiencies.
The growth of LNG projects with third-party pipeline feed
gas, coupled with lower LNG prices and an increased awareness
of CO
2
emissions, have caused project owners to focus more on
plant efficiency improvements and embrace the use of
innovative technology to improve project lifecycle economics.
This article looks at how purposefully designed mid scale LNG
trains with combined cycle heat integration can increase LNG
plant efficiency and reduce carbon emissions while remaining
highly cost-competitive.
LNG plant energy efficiency
LNG plants convert feed gas into a chilled liquid product
after removing the contaminants from the natural gas. LNG
plants use part of the feed gas as fuel to produce the required
energy to provide refrigeration, gas treatment, plant heating,
boil-off gas (BOG) handling, ship loading and other utilities.
The plant efficiency is therefore calculated by the ratio of the
heating value of product to the heating value of the feed gas,
which typically ranges from 88% to 92% for LNG export plants
located close to the equator.
LNG liquefaction efficiency is primarily influenced by two
factors: heat exchange efficiency, and the efficiency of
turbomachinery. Heat exchange efficiency depends on the
refrigerant selection, liquefaction process configuration,
exchanger type and temperature approach. Turbomachinery
efficiency depends on the selection of gas turbines and
compressor string arrangement. A considerable amount of
optimisation has been undertaken in traditional export scale
liquefaction processes to maximise the heat exchange
efficiency by optimising the cooling enthalpy curves. Similarly,
the move to aeroderivative gas turbines over industrial frame
gas turbines coupled with refinement of refrigeration
compressors have seen gradual efficiency improvements in
turbomachinery over the years of a few percentage points.
LNG plants can also utilise heat integration and boil-off
recovery to reduce fuel consumption and improve energy
efficiency. Large scale LNG plants have often implemented
limited sensible heat recovery systems from the compressor
drive gas turbines exhausts. The most common method is to
use a heat transfer medium, hot oil or water, to recover sensible
heat from the gas turbine exhausts to be utilised in the acid
gas removal unit, molecular sieve dryer regeneration, and in
fractionation units. The use of heat recovery steam generators
(HRSGs) to recover waste heat from the gas turbine exhausts
has been implemented in a few LNG plants, such as in
Tangguh LNG,
1
Qatar LNG Trains in Ras Laffan
2
and recently in
Cove Point LNG. This article looks at heat recovery arrangement
where waste heat from gas turbine driving the mixed
refrigeration loop is utilised to drive a steam turbine that
powers the precooling refrigeration circuit, thus forming a
combined cycle refrigeration system. A significant increase in
LNG plant efficiency can be achieved when such a combined
cycle refrigeration arrangement is utilised in LNG plants.
Combined cycle power plants
Gas turbine power plants have adopted combined cycle power
generation since the 1960s to increase efficiency. Initially,
HRSG units provided steam for co-generation applications,
since the gas turbine temperature level was still relatively
low. In the late 1960s, the gas turbine unit sizes became
large enough to start building combined cycle power plants
with HRSGs supplying the main steam for a bottoming steam
turbine cycle. Cogeneration is also referred as combined heat
and power, which provides electric power and process steam
with higher fuel efficiency. The introduction of Public Utilities
Regulatory Policy (PURPA) in 1978 had promoted the use of
cogeneration plants in the US.
3
With advancing gas turbine
technology, combined cycle power plants reached net efficiency
levels closer to 60%, meeting US Department of Energy (DOE)
efficiency targets in the 1990s. An energy distribution schematic
of a typical combined cycle power plant is provided in Figure 1.
Combined heat and power plant
Heat integration and combined heat and power forms the
central part of the patented OSMR® LNG process. Waste heat
from the mixed refrigerant compressor gas turbine exhaust is
utilised in steam generators to produce high pressure steam,
and then used to drive the precooling refrigerant compressors
and meet plant heating requirements. A process schematic is
shown in Figure 2.
Combined cycle configuration in an LNG plant should
avoid complex steam systems to ensure capital costs
associated with steam systems remain minimal and enable
plant operators to focus on LNG production operations. The
Figure 1.
Energy distribution in a combined cycle power
plant.