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LNG SPECIFICATIONS
Natural gas interchangeability in focus as sources of LNG widen David Wood and Saeid Mokhatab explain how national quality specification guidelines with respect to LNG imports have yet to be fully resolved from the contract awards to deliveries of natural gas conventional natural gas supply in the
Natural gas interchangeability is a common issue for distribution networks
US is not capable of meeting the gas
35 different compositions of gas ENTERING7ESTERN%UROPE¸SPIPELINENETWORKS
to address in major markets where supplies are coming from many different
demand, the
(Figure 1) supplying the market. Certain countries, such as Belgium, France, Germany and the Netherlands, operate both a high-calorific and a lowcalorific
gas
pipeline
1400
1300
LNG 0IPELINE
1200
distribution
US
Wobble Index (MJ/m3)
different imported LNG gas compositions
gas
compositions,
Estimates indicate that by 2010, 10 percent of US demand could be met by LNG. Many participants in the US market have called for pipeline standards to
clarify
issues
surrounding
gas
interchangeability to ensure consistently safe and reliable sources of supply.
The gas supply chains in these countries
have
evolved
in
1100 0.55
an
0.6 2ELATIVEDENSITY
environment where interchangeability is an ongoing issue that had to be managed and a range of parties along the supply
0.65
LNG imports
0.75
Many gas liquefaction plants are located in remote locations far from natural gas
Figure 1: Source gas entering pipeline networks in Western Europe. Interchangeability is has to be resolved when new source enters market
liquids
chains had to agree compositional regulations
to
cope
with
this
compositional diversity. In contrast, North America and the Kingdom,
where
grid
gas
relative
low
calorific
gas
and
(NGL)
markets,
making
it
commercially unacceptable to separate compositions
forms
a
significant
an
the lighter NGL components from the
component of gas supplied into these
interchangeability index calculation) to
gas, especially ethane (C2) and in some
markets. Gas tariffs in these US states
establish a specification that directly
cases propane (C3) and butane (C4).
are based on such gas specifications.
relates to the performance of a gas
specifications have historically preferred a
pipeline
therefore, will become less homogenous.
network.
United
will
(e.g. coal bed methane).
Wobble Index (BTU/SCF)
qualities of pipeline gas and about 15
mix
smaller non-conventional gas sources
In continental Europe, for example, there have been about 20 different
supply
increasingly involve imported LNG and
1500
sources of varying qualities.
US
density
(as
is
done
in
burner.
Wobbe Index
Some NGLs are liquefied along with the methane. Indeed, Japanese LNG
The Wobbe Index adjusts the heating
buyers that have dominated the LNG
compositions have been dominated by
The current specifications found in most
value for the relative density of the gas. It
markets for decades prefer high calorific
domestic
production
the
US pipeline tariffs are based upon the
is internationally the most widely
gas and often choose to add C3 and C4 to
network
through
gathering
overall heating value (or gas calorific
accepted measure of interchangeability.
boost the NGL and energy content of the
networks, are only recently having to
value, GCV) of the gas. All networks
From the lower heating value (LHV)
come
fluctuating
specify a minimum and maximum for
in Btu/scf [kJ/Nm3] and the specific
Much LNG produced is therefore
compositions from a range of import
heating value. For heating value, existing
gravity (SG), the Wobbe Index (WI) of the
richer in NGLs than the US and UK gas
sources in small but significant batches.
networks consist of three major groups.
gas can be calculated as the LHV divided
grid specifications.
Hence, gas interchangeability is yet to
Asia (Japan, South Korea, Taiwan),
by the square root of the relative density:
be fully resolved in terms of national
where the distributed gas is rich, with
quality specification guidelines with
a heating value greater than 43
respect to LNG imports and non-
million Joules/cu m (1,090 btu/scf).
conventional gas sources in both US and
UK and US, where distributed gas is
This index is frequently used as a
lean, with a heating value usually less
parameter, particularly in the UK, for
This results in regasified LNG having
to
terms
entering
large with
the UK. Many international gas markets have
gas derived from the delivered LNG.
On the other hand, most of the inert gases such as nitrogen and corrosive acid
WI = LHV / √SG
gases such as carbon dioxide and hydrogen sulphide are removed prior to gas liquefaction.
than 42 million Joules/cu m (1,065
which the upper limit constitutes a major
a higher heating value than US pipeline
adopted interchangeability parameters to
btu/scf).
constraint on the import of rich gases.
gas, where the ethane is routinely
ensure end-user protection while dealing
Continental
with multiple supply sources. Practically all global gas markets (and even some regions of the US) have
Europe,
the
Other limits for main gas components
stripped from the gas stream and sold to
acceptable heating value range is
are a minimum amount of methane, and
process gas customers (e.g. petrochemical
quite wide—about 39-46 million
maximum amounts of ethane, propane,
plants) in the main gas supply regions
Joules/cu m (990-1,160 btu/scf).
butane, pentanes and heavier, and inert
(e.g. Gulf of Mexico) and the inert
where
adopted the use of interchangeability
However, heating value does not address
gases, particularly nitrogen, oxygen, and
components and more carbon dioxide are
parameters in their contractual terms
the burner performance of the gas, just
carbon dioxide.
left in the gas.
and conditions.
its energy content.
Most US gas specifications are based
There is, therefore, a clear discrepancy
The most commonly used reference is
It is burner performance that many
upon heating value, not Wobbe Index.
between the acceptable heating value
the Wobbe Index. Indeed, California,
end users are primarily interested in and
This has historically been adequate
ranges for Asia and the US. This is also
Wyoming and Colorado, have long
adjust their combustion equipment to
because most gas supplies came from
true for the Wobbe Index in Japan and the
established
optimise that performance.
interstate transmission pipelines with
UK. This is the main challenge for LNG
very homogeneous gas compositions.
producers wishing to sell to the Asian, US,
specifications
for
interchangeability, primarily because local
14
gas
•
production
LNG journal
of
variable
There must be an adjustment of the heating value of the gas for its relative
• The World’s Leading LNG journal
However, in the future as the domestic
and UK markets at the same time.
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LNG SPECIFICATIONS
Existing LNG receiving terminals in
Only LNG from Trinidad & Tobago
the US pipelines translate their current
LNG supplies throughout the world
the US have been feeding gas derived from
could be directly delivered into most gas
heat
a
(Wobbe Index generally in the range 1380
LNG into the existing pipeline system for
markets in 2005 along the East Coast of
corresponding Wobbe Index (generally in
–1440 BTU/SCF), when blended with
a number of years, but in relatively small
the US using such criteria. However, if
the range 1330 –1370 BTU/SCF), most
allowable concentrations of an inert gas
content
specification
into
quantities compared to other domestic sources of supply. Such gas has raised some concerns
about
its
suitability and reliability as
we pro your in tect sulatio n
fuel for some large gas consumers, especially for gas turbines using dry-lownitrogen oxide combustion systems, which have to burn
engineered solutions
below threshold temperatures to be effective.
Foster
LNG quality options Many US gas pipelines which are in positions to take
gas
terminals
from have
LNG heating
value limits of 1050 -1070 BTU/SCF (HHV). Imported LNG from many sources have heating values from 1080 -1160 BTU/SCF
®
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(see table). To import such LNG and comply with gas send-out requirements, the heating value must somehow be reduced. The
most
common
approach is to inject inert gas (e.g. nitrogen) up to the pipeline
limit
for
inert
content, usually 2 percent to 3 percent. Air
injection
is
also
technically feasible, but is only
used
for
adjustments
of
minor heating
Tembutil-IF™ vapour barrier foil
superior-grade butyl with a reinforced, impermeable aluminium foil excellent adhesion, flexibility and workability in hot and cold conditions flame retardant - M1 certified ‘fingerlift’ release foil, easy to peel in cold conditions or with gloves
value to avoid the gas exceeding the minimum free-oxygen
content
specifications, which are 0.01-0.2 percent. The
BTU
reduction
possible by adding inerts alone is limited to about 2030 BTU/SCF. This may be sufficient to adjust quality for some LNG streams, but will be insufficient for other streams. Under the current heat
Fibaroll
U.V.-hardening polyester foil tough and durable, providing extreme mechanical resistance available in fire-retardant version easy to apply - requires only ultra-violet (sun)light to cure no need for metal jacketing
content-based pipeline tariff specifications common in the US few supplies of imported LNG meet the restrictive provisions.
gas
quality
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LNG journal
•
February 2007
•
15
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LNG SPECIFICATIONS
such as nitrogen, would meet the tariff specification for interchangeability (on
section. The composition of suitable turbine fuel
that Wobbe Index basis).
Hydrate production is influenced, in
A superheat of at least 50ºF (28K) (an
turn, by gas composition; gas temperature,
industry standard) over the dew point is
varies,
from
gas
with
gas pressure and the pressure drop in the
required to ensure that no liquid dropout
amounts
of
NGLs
gases
and
gas fuel system. Liquid water in the
occurs in the fuel system components
US heating value constraints to reduce
heavier hydrocarbons to pipeline gas
presence of H2S or CO2 forms acid gases
downstream of the heater.
the heating value, the C2 and heavier
consisting mostly of methane, to lean
that can attack and corrode fuel supply
components need to be removed at extra
gas with significant amounts of non-
lines and components.
cost to the LNG receiving terminal.
combustible
In cases where inerts cannot meet the
significant
A standard gas analysis alone may not be entirely sufficient for the detection of
Free water can also cause turbine
heavy hydrocarbons, because it may only
flameouts or operating instability if
include the gases, but not the liquids in
ingested in the combustor or fuel control
the stream.
voiced
components. LNG is unlikely to be
Also,
investment and operating costs than
concerns about gas quality fluctuations
responsible for free water as water is also
aggregate all hydrocarbons from hexane
systems that involve injection of inerts or
in pipeline gas.
a problem for the liquefaction process.
and heavier into one value in such
Systems to reduce C2+ or C3+ gases at LNG receiving terminals can be more cost
effective
in
terms
of
capital
boosting gas sendout pressure to achieve
components
such
as
nitrogen or carbon dioxide. In recent years some gas turbine operators
in
the
US
have
Two possible causes are likely: (1)
heating value constraints, according to
insufficient
NGL
published papers.
processing
plants
extraction due
to
at
Heavy hydrocarbon gases present as
it
is
common
practice
to
analysis and not record trace contents of
gas
liquids in the fuel gas provide many
of
times the heating value per unit volume
While this is perfectly acceptable for the
lack
heavier hydrocarbons.
A benefit of C2+ removal solutions is
commerciality for LPG products, resulting
than they would if maintained in a
calculation of the lower heating value
the production of a high-value NGL
in high calorific value gas with high
gaseous state.
(LHV) as long as the hexane and heavier
stream at the LNG receiving terminal,
hydrocarbon dew points; (2) introduction
Since turbine fuel systems meter the
hydrocarbons constitute a minute fraction
which can significantly enhance the
into a pipeline grid of slugs of imported
fuel based on the fuel being a gas, this
of the gas, it can lead to an incorrect
overall economics of the terminal.
high NGL LNG-derived gas with low
creates a safety problem, especially
estimate of the hydrocarbon dew point.
The NGL removal could be conducted on
hydrocarbon and water dew points and a
during the engine start-up sequence
The Gas Processors Association (GPA)
regasified LNG by additional processing in
low carbon dioxide content compared to
when the supply line to the turbine is
amongst others recommended in 2005
a cryogenic expander processing plant
the usual pipeline supply of domestic gas.
cooler. Hydrocarbon liquids present in a
that the determination of hydrocarbon
downstream of the terminal.
Gas derived from LNG produced from
fuel gas can cause a range of problems for
dew point should be done using extended
domestic pipeline gas for peak shaving
gas turbines (Kurz, 2005), including:
gas analyses (C9+) combined with
purposes is unlikely to cause such gas
Turbine over-fuelling, which can
equation of state calculations to overcome
However, it is less costly and more efficient
to
integrate
fractionation
columns into the receiving terminal and remove C2+ (or C3+) components from
interchangeability problems. The
combustible
cause an explosion or severe turbine
components
in
problems,
other important combustion parameters
molecular weight hydrocarbons (NGLs),
because the system gain will vary as
regarding gas interchangeability, such as
hydrogen and carbon monoxide.
liquid slugs or droplets move through
the sooting index, incomplete combustion
the control system.
factor, yellow tip index, and flashback,
the LNG before vaporization (Yang et al.,
natural gas consist of methane, other low
2003; Price et al., 2005).
Turbines, fuel quality Industrial gas turbines, a key consumer of
Free water and heavy hydrocarbons as
Combustor
performance problems for
subsequent
gas turbines. Higher
operation with a
concentrations
wide variety
of water and
of
In recent years some
acid gases
gas turbine operators in the US have voiced
which need to be considered from
section
specification and consumer perspectives.
Over-fuelling the bottom section of
may for example be designed for a Wobbe
the combustor when liquids gravitate
Index of 1220 ±10% Btu/scf (48,031 ±10%
towards the bottom of the manifold.
kJ/ Nm3) based on the LHV of the fuel.
streaks
engine
hot
damage.
Different gas compositions could yield
when trapped liquids pyrolyze in the
the same Wobbe Index, but they may have
hot gas passages.
widely different hydrocarbon dew points.
Liquid carryover is a known cause for
Gas fuel supply and package lines
rapid degradation of the hot gas path
may need to be heat-traced to keep the
components in a turbine.
gas fuel supply above the gas dew point
turbine fuel gas.
increase corrosion problems
A standard gas turbine fuel system
Internal injector blockage over time,
hydrocarbon gas liquids present in the
in pipeline gas.
during periods when the engine is not operating. Minimum engine flameout fuel flows will also vary if the fuel contains high
If such problems are being experienced
percentages of non-combustible gases. A
then it follows that more maintenance
turbine expected to operate with gaseous
attention of the turbine may be required
fuels exhibiting a wide Wobbe Index
to assure that combustion components are
range
in premium condition.
differently and more rigorously than one
This may require that fuel nozzles be
will
need
to
be
configured
that will only operate with a small variance in Wobbe Index.
(also
inspected and cleaned at more regular
an issue for pipelines
intervals or that improved fuel filtration
The fuel supply contract should ideally
and underground gas storage
and fuel treatment components be
specify the allowable variations in
installed at additional cost.
composition and temperature.
liquid
fuels. However, the quality
and
hot
lead to such problems related to heavy
quality fluctuations gaseous
stability
NGL rich gas derived from LNG could
concerns about gas
and composition of fuel burned in
control
liquids are the main cause of combustion
natural gas, do allow
and
Fuel
this problem. The composition of a gas also affects
damage.
facilities). Water in the gas may combine with
Protection against heavy gases and
expectancy and maintenance
other small molecules to produce a
water present as liquids can be achieved
US policies
requirements, particularly its
hydrate – a solid with an ice-like
by heating the fuel downstream of
Many in the US industry have called
combustion system and turbine
appearance.
knockout drums and coalescing filters.
in the past two years for the adoption
a gas turbine impacts its life
16
•
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• The World’s Leading LNG journal
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LNG SPECIFICATIONS
of
nationwide
and
interchangeability
gas
quality regulatory
the best approach at the present time is
informed by a statement of policy on
to the Commission’s jurisdiction should
to
these issues”.
include in its tariffs, specific terms and
proceed
case-by-case,
with
the
specifications, including such bodies as the
Commission’s action in dealing with gas
Further FERC states (June 2006)
conditions of service that address gas
Natural Gas Supply Association. Four key
quality and interchangeability issues
“that every natural gas company subject
quality and interchangeability issues. In
specifications are typically referred to in such calls: Cricondentherm hydrocarbon dew point (CHDP) Maximum Wobbe Index Maximum Inert Maximum Butanes Plus (C4+) The NGSA’s 2005 petition requested
that
FERC
establish: (1) a minimum “safe harbor” national CHDP limit
of
15
Fahrenheit
to
degrees apply
at
pipeline delivery points; (2) require pipelines to adopt interchangeability standards of a maximum 1400 Wobbe Index number; a maximum 4 percent inert gas limit; 1.5 Plus
and;
(3)
require
pipelines to implement tariff language implementing a non-discriminatory harbor
safe
mechanism
establishing
the
use
© 2005 Swagelok Company
percent maximum Butanes
of
aggregation, blending and the contractual “pairing” of natural
gas
volumes
of
differing qualities for the purpose
of
blending
You need to get to market quicker but paperwork is holding you back.
to
achieve a desired quality. Many bodies commented
And the pressure is on.
as part of the consultation process
preferred
alternative values or points of application and many from upstream, midstream, transportation
and
consumer sectors of the
Swagelok understands the demands made on you. We provide the
industry aligned with the
solutions you need that are approved to industry standards such as
more
values
ECE R110. For example, our alternate fuel service 2-way ball valve
included in the Natural Gas
meets and exceeds the market’s temperature and pressure require-
flexible
Council
(NGC)
guidelines:
interim
for
example
range of plus and minus four
percent
historical
of
Wobbe
number
subject
local
ments, it also has the necessary approvals to reduce documentation delays. Add to that, our industry-specific expertise, global sales and support network, broad product range and our proven manufacturing
Index
excellence, and you’ll realize we’re the only alternative when you want
to
to move swiftly from design to marketplace. Contact your authorized
a
maximum of 1400 and a
Swagelok sales and service center or visit www.swagelok.com
minimum of 1200 to provide some
flexibility
in
the
standards. In (June,
any
event
2006)
FERC decided
against NGSA rulemaking petition and stated “that
LNG journal
•
February 2007
•
17
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LNG SPECIFICATIONS
essence, pipelines should adopt gas
in ethane, butane and propane, while
quality and interchangeability standards
hexane and other heavy hydrocarbons
in their tariffs that are modeled on the
are virtually absent.
The NGC is an organization made up of the representatives of the trade
specification to satisfy all potential
Terminal operators can use a range of
customers in Asian, European or North
processes to adjust gas quality (add
American gas markets, which would
The Wobbe Index and BTU contents
inerts or LPG or remove LPG) and have
require additional capital investment
are therefore higher than for typical
opportunities to blend batches of LNG in
and operating costs, or to produce LNG of
pipeline quality gas, but the dew points
their storage tanks prior to shipping gas
only one quality and potentially limit
are usually lower.
into pipeline networks downstream.
their marketing opportunities.
NGC+ interim guidelines or explain how their tariff provisions differ”.
for the LNG qualities it can accept.
associations of the different sectors of the
The fuel gas characteristics that are
A terminal operator's ability to handle
Clarity of gas specification throughout
natural gas industry, such as the
relevant for the combustion process in
an LNG with a borderline quality
the US gas network and in supply
producers,
local
the LNG derived gases shown (i.e. Wobbe
without adversely impacting the quality
contract, incorporating Wobbe Index
distribution companies (the plus refers to
Index, dew point and flame temperature)
of the sendout gas will vary according to
numbers and dew point ranges, would
other bodies from the industry that
are well within the normal range that a
whether it is a single spot cargo or one of
undoubtedly
aligned with them). One of FERC’s
well-designed
a series of multiple cargoes delivered
terminal operators manage their LNG
concerns was that “generic application of
system of a gas turbine can handle.
under a long-term contract with a specific
inventories and would also seem to be in
liquefaction plant.
the interest of most of their gas
pipelines,
and
fuel
and
combustion
interchangeability
When LNG is shipped in tankers or
specification would appear to require
stored in tanks for long periods, its
potentially significant and industry-wide
composition changes slightly due to an
Conclusions
changes from the historic gas quality
ongoing boil-off process.
The diversification of international LNG
NGSA’s
proposed
experienced in different regions.”
supply chains means that LNG producers
entering through the tank insulation.
now plan to supply a range of countries
More
through
that its gas will be accepted for
nitrogen and methane evaporate faster
destinations
transportation by the pipeline absent
than the heavier components such as C2
liquefaction plant.
extraordinary circumstances if its gas
and C2+.
components
such
as
deliveries from
at
multiple
each
specific
In the past, most liquefaction plants
Evaporation therefore increases the
were designed to serve clearly identified
As natural gas is transported and
gas heating value. If an LNG delivery is
markets in long-term supply contracts.
distributed, it may experience changes
rich in heavy components with low
These
in temperature and pressure, which
nitrogen content, this aging effect will
particular LNG quality parameters that
cause heavy hydrocarbons in the gas
increase the heating value even more.
the plant had to meet.
conforms to the specification.
It is the composition of delivered LNG
stream to assume a liquid form (i.e. or
liquid dropout). When this happens, pipelines and other
downstream
equipment
and
customers may experience inefficient
blended
LNG
in
Increasingly
specified
different
the
gas
specifications and standards in the expanding destination gas markets
the
mean that the quality of the LNG they
produced
at
the
liquefaction plant.
deliver is more important than when the
storage
periods
are
extended
this
Problem
compositional change has to be taken into
The potential for this problem to occur
account
can be measured in terms of criconden-
composition of the sendout gas.
therm hydrocarbon dew point (CHDP).
also
receiving
If transportation distance is long or
operations and unsafe conditions.
contracts
terminals tanks that is important, not composition
the
when
establishing
the
industry consisted of simple linear supply chains. Delivery of significant quantities of high heating value LNG produced from rich
gas
into
receiving
terminals
It is unlikely that this process would
connected to low-calorific-value gas
with
change the heating value by more than
pipeline networks is a challenge for both
temperature and pressure, and therefore
about 1 percent, but such a change could
ends of the LNG supply chain.
can vary daily or within a single day,
be significant if it pushes the sendout gas
presenting producers with significant
into “off-spec” compositions.
However
commercial gas
will
CHDP
varies
uncertainty meet
very
that narrow
their
Gas composition in a pipeline system
gas
fed predominantly by LNG-derived gas may see significant swings if batches
specification ranges. CHDP ranges of 10oF to 40oF provide
coming from different LNG cargoes from
more flexibility than a rigid value
different origins are fed sequentially into
of 15oF.
that system.
Even though the NGSA proposed
Such instability in gas composition is
standard has not been adopted, these
problematic for gas turbines. Some “Dry
variables as described in the NGC
Low Nox” systems that meet typical
guidelines will clearly influence contracts
emissions requirements allow swings in
and tariffs involved in gas sales contracts
the Wobbe Index of up to 10 percent,
going forward, as they do many existing
while other designs may be limited to 2
contracts, but flexibility is retained by
or 3 percent swings.
avoiding the nationwide application of a
This is a key reason for concern about LNG terminals as a source of gas supply
rigid standard. LNG compositions are close to typical
among gas turbine operators.
compositions found for US pipeline
In practice, each LNG terminal
quality gas, but with lower methane
agreement can involve negotiations that
contents and somewhat higher contents
result in slightly different specifications
18
•
LNG journal
• The World’s Leading LNG journal
receiving
particularly power generators.
Some LNG evaporates due to heat volatile
LNG
customers throughout the supply chain,
remains fixed and ensures a producer
A “safe harbor” standard is one that
help
LNG
producers
have
to
decide
whether to produce more than one LNG
David Wood is an international energy consultant specialising in the integration of technical, economic, risk and strategic information to aid portfolio evaluation and management decisions. He holds a PhD from Imperial College, London. Research and training concerning a wide range of energy related topics, including project contracts, economics, gas / LNG / GTL, portfolio and risk analysis are key parts of his work. He is based in Lincoln, UK, but operates worldwide. Saeid Mokhatab is an advisor of natural gas engineering research projects in the Chemical and Petroleum Engineering Department of the University of Wyoming, USA. His interests include natural gas engineering, a particular emphasis on natural gas transportation, LNG, CNG and processing. He has participated in several international projects related to his areas of specialization, and has published more than 50 academic and industrial oriented papers and books. Mokhatab served on the Board of SPE London Section during 2003-6, and is currently a member of SPE Distinguished Lecturer Committee, SPE Distinguished Achievement for Petroleum Engineering Faculty Award Committee, and several other professional bodies.
References Bramoullé, Y., Morin, P., and Capelle, J-Y., “Differing Market Quality Specs Challenge LNG Producers”, Oil & Gas Journal, 102, 38, 48-55 (October 11, 2004). Federal Energy Regulatory Commission, Natural Gas Supply Association Docket No. RM06-17-000 Order Denying Petition For Rulemaking (Issued June 15, 2006). Kurz, R., “Determining Gas Fuel Quality”, Gas Machinery Research Council’s Gas Machinery Conference, Colorado,
USA (Oct. 2-4, 2004). Price, B., and McCartney, D., “LNG Terminals - Concepts for Increased Efficiency and Economic Enhancement”, paper presented at the 84th GPA Annual Convention, San Antonio, TX, USA (March 13-16, 2005). Yang, C.C., Kaplan, A., and Huang, Z., “Cost Effective Design Reduces C2 and C3 at LNG Receiving Terminals, Oil & Gas Journal, 101, 21 (May 26, 2003).
