Modern Gas Turbines with High
Fuel Flexibility
Volker Poloczek and
Heinrich Hermsmeyer
Siemens AG, Energy Sector
Germany
POWER-GEN Asia 2008 – Kuala Lumpur, Malaysia
October 21-23, 2008
Copyright © Siemens AG 2008. All rights reserved.
Copyright © Siemens AG 2008. All rights reserved.
Abstract
Siemens experience with fuels combusted in heavy duty gas turbines is based on the existing
Siemens and former Westinghouse fleet. Commercial and environmental requirements were
successfully met.
These gas turbine product lines are capable of firing of natural gas, fuel oil, syngas, low-BTU
gas, naphtha, crude oil and other fuels. This covers fuel applications beyond the standard
heating values (LHV for gas = 48MJ/kg; LHV for oil = 42MJ/kg) and their corresponding
properties.
This paper focuses on the major non-standard fuels.
Physical parameters such as specific reactivity, Wobbe index, flash point, viscosity or
impurities must be determined for these fuels. Fuel analyses are used to derive system
adaptations based on proven design features.
Heavy duty gas turbine applications for gaseous fuels have been developed and are in service
for a broad range of different fuels and different markets around the world. The E- and Fclass essentially target the 50 & 60 Hz-market for power, and combined heat and power
applications. Mechanical drive solutions are also available for the oil & gas sector.
Siemens has successfully applied a broad range of different fuels as mentioned above over
the last 40 years.
Ongoing developments based on market requirements are focusing on syngas for IGCC, CO2
capture & storage, polygeneration, for example as well as on "fuel-upgrades" for the service
fleet.
These developments in conjunction with combustion test rig validations enable us to fulfill
customer needs to meet future requirements for even greater fuel flexibility.
On this basis, Siemens provides its new apparatus and service business customers around the
world with high-benefit gas turbine solutions world wide.
Copyright © Siemens AG 2008. All rights reserved.
Table of Contents
1
Market requirements............................................................................................................4
2
Fuel applications..................................................................................................................5
2.1
Liquefied natural gas (LNG).......................................................................................5
2.1.1
Overview and features of LNG...............................................................................5
2.1.2
Gas turbine operation with LNG...........................................................................10
2.2
Syngas .......................................................................................................................11
2.2.1
Overview and features of Syngas .........................................................................11
2.2.2
Application of Syngas...........................................................................................14
2.3
Crude oil....................................................................................................................15
2.3.1.
Overview & features of crude oil......................................................................15
2.3.2
Application of crude oil ........................................................................................16
3
Developments for tomorrow..............................................................................................17
4
Literature ...........................................................................................................................18
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1
Market requirements
The increasing demand for energy and the continuing increases in prices for standard fuels
demand greater flexibility in the use of fuels in gas turbines.
Besides the standard fuels natural gas (typical heating values between 39 to 46 MJ/kg) and
Diesel No. 2 fuel oil (42 MJ/kg), there is increasing interest in low-BTU gases, synthetic
gases (syngas here) and even liquid fuels (e.g. heavy fuel oil, Naphtha and condensates).
Low-BTU gases refer to fuels with heating values between 10 and 35 MJ/kg. Syngas denotes
synthetically produced gases that generally have even lower heating values, between 4 and 12
MJ/kg.
The graphic below (see Fig. 1) uses two scenarios based on the change in the price of oil to
show the share of various fuels used in the installed gas turbine base in the year 2020.
In 2005 the standard fuel gas was the major fuel for the installed gas turbine fleet, with a
share of about 86%. This includes LNG which basically has properties close to those of
natural gas.
Liquid fuels and non-standard fuels are the second major group here with a relatively small
share of about 14%.
But the non-standard gaseous fuels such as low-BTU fuels or syngas show the highest rate of
growth by 2020, increasing by a factor of 6.
Fig. 1: Shares of non-conventional fuels and liquefied natural gas increase substantially.
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This is driven by the oil & gas business which is interested in firing tail or waste gases (such
as methane admixed with inert gas) from the LNG / GTL process. Their target is to "refine"
the natural gas resources into high-end products such as clean liquid fuels, lubricants or LNG
to give high value-added. Siemens has therefore developed the so-called Compressor Drive
GT (CD) and the E-LNG GT based on well-proven Siemens GT fleet technology for the
SGTx-2000E. These gas turbines are fired with a low-BTU gas and offer flexibility to fulfill
the demanding requirements of the oil & gas business.
Syngas is the other major driver in the newly emerging market business. The reason is the
increasing interest in greater fuel flexibility, and CO2 sequestration and storage to support
clean environment targets.
Nevertheless, market requirements and developments for non-standard liquid fuels – e.g.
crude oil - also apply in the context of greater fuel flexibility for gas turbines per se.
Siemens already has extensive operating experience with a broad range of proven and reliable
gas turbines. The first major criterion for successful use of fuels is the combustion stability,
with appropriate consideration of emission limits and guaranteed values to be met with
respect to output and efficiency. The second is the validation of proper combustibility of the
requested fuel types during development and prior to the delivery of the gas turbine to
customer’s site. This ensures highest confidence in the combustibility of these non-standard
fuels.
Siemens maintains an ongoing dialog with operators and potential customers around the
world while investigating fuel applications for the existing fleets.
2
Fuel applications
The abstracts which follow give a short overview of the various fuels of interest, their special
features to be considered for their application as well as Siemens' operational experience. The
sequence in which they are listed is essentially based on the frequency of their application.
2.1
Liquefied natural gas (LNG)
2.1.1
Overview and features of LNG
Liquefied natural gas is becoming an increasingly important source of energy throughout the
world. Even in countries with an existing infrastructure (international gas supply agreements
Copyright © Siemens AG 2008. All rights reserved.
with existing pipelines), the use of liquefied natural gas to increase capacity is either under
consideration or corresponding infrastructure modifications are being initiated. Current
capacities in these countries are already at their limits, and increases in capacity are now to be
realized in liquid form via liquefied natural gas.
As a result, this fuel gas serves two fields of business simultaneously:
•
LNG consumption: Combustion of LNG in gas turbines for electricity and heat
production
•
LNG production: Gas turbine used in a LNG plant for mechanical and electrical
power production, with this turbine firing tail / waste gases.
LNG consumption:
Market research indicates that the market share of liquefied natural gas will increase
substantially in the coming years, from approximately 140 megatons per annum at present to
approximately 500 megatons per annum in 2015.
Fig. 2: Compositions of liquefied natural gas sources.
As a result of its production process, liquefied natural gas differs from pure natural gas in a
number of properties. Potential low-calorific constituents are volatilized by the liquefaction
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and deliquefaction process. Liquefied natural gas is converted to the gaseous state prior to
combustion in the gas turbine and can thus be treated as a gaseous fuel as regards both
systems and combustion.
The production process results in changes in the gas composition, with increased fractions of
higher hydrocarbons and higher heating values. The heating values can be reduced by the
targeted addition of inert fractions such as nitrogen (N2) during regasification.
However, for combustion the heating value is less relevant than the Wobbe index. The
Wobbe index relates the heating value of a gas to its density and is thus a combustionrelevant variable for burner capacity.
The table in Figure 2 shows typical liquefied natural gas compositions for higher C-fractions
and their heating values and Wobbe indexes. The deviation from pure natural gas is less than
ten percent, but even these slight variations can cause combustion instabilities if no
modifications are implemented. With an unmodified combustion system, the elevated heating
values may result in a slight increase in emissions.
LNG production:
The second field of business served is LNG production. Here the gas turbine fires low-BTU
fuel gas (tail or waste gas) to drive the process compressor(s) or to generate electrical energy
for an LNG plant with electric drives (E-LNG). Both applications are served with the same
GT, which generates heat as well as process steam in the HSRG and is based on the wellproven SGTx-2000E.
LowBTU gases are mainly composed by methane and inserts (N2, CO2, see yellow back
ground).
Consequently the growing interest in LNG is served by gas turbine technology from both
sides. The use for “LNG production” as well as for “LNG consumption” shows again the
high fuel flexibility of Siemens gas turbines.
The permanently carried out operator- and customer- interviews by Siemens marketing &
sales guided to Based on continuous customers’ feedback Siemens initiated an upgrade
program which especially considered the demanding requirements of the Oil&Gas business.
Here the focus is put – beside operational topics – this program is focusing on the combustion
validation of inert-gas rich LowBTU fuel gases in combustion high pressure tests.
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Fig. 4: Flow chart of development for high-benefit combustion system.
The high pressure (HP) combustion tests are required to validate the separate fuels with their
fractions (see Fig. 5). The test program therefore considers a broad band of low-BTU fuels
bounding the design value.
CH4 vol%
N2
vol%
other vol%
Fuel 1
86
9
5
Fuel 2
75
20
5
Fuel 3
66
30
4
Fuel 4
56
40
4
Fig. 5: Low-BTU fuel gas composition successfully validated in HP test.
One major question with regard to combustibility for such low calorific fuel gas is the
combustion stability close to lean blow-off.
The first indicator for the stability limit is the measured CO in the exhaust gas. The "sudden
CO increase" in premix combustion mode is a first indicator, but shows that the stability limit
has not yet been reached. In the described combustion HP tests CO increases to some 100
ppmvd without any degradation of stability (see Figure 6).
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Fig. 6: Successful stability limit validation in HP combustion test as a function of N2
The stability limit is a function – indicated by a rapid increase of CO-emission in premix
mode – of the inert-gas fraction in the fuel gas. To compensate this effect for low-BTU fuel
gas, which has a high inert-gas faction, the flame-stabilizing pilot gas component has to be
adjusted.
Fig. 7: Emissions with low-BTU fuel gas for the SGTx-2000E
The other important combustion topics are emissions like NOx and CO. The target was to
achieve low emissions over a broad operating range for the GT. To this end, part-load as well
as base-load combustion tests were performed. They show – as documented in Figure 7 – that
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NOx emissions are below 25 ppmvd at 15 % O2 dry. Associated CO emissions measured in
the operating range were below 10 ppmvd at 15 % O2 dry.
The next step is targeting inert gas fractions beyond the above-mentioned range to make the
gas turbine even more flexible for the future requirements set by world-wide customer
demands.
2.1.2 Gas turbine operation with LNG
Operation of the gas turbine from start to part-load and base load and vice versa is the same
here as for a standard simple cycle or combined cycle operation (see Figure 8). In certain
cases a standard fuel is used for startup and shutdown.
Fig. 8: Gas turbine operating line for low-BTU gas and standard fuel
Upstream online gas analysis is used to safeguard gas turbine operation in the face of
changing gas qualities for liquefied natural gas sources. A downstream control loop adjusts
the pilot gas volume that stabilizes the flame in premix mode.
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Fig. 9: Operating principle for Wobbe index control device
The necessary device is called "Wobbe index control" (see Figure 9) and is located on the
fuel gas package upstream of the burner. It performs online monitoring of the lower heating
value, fuel density and some major constituents of the supplied low-BTU gas. These data will
be used to derive the Wobbe index in the control system to the Wobbe number and the result
compared to the required Wobbe range. If the allowable range is not fully met, pilot gas
adjustments will be initiated automatically to ensure proper operation of the gas turbine.
2.2
Syngas
2.2.1
Overview and features of syngas
Siemens – and the former Westinghouse gas turbine division – have a very long history of
syngas experience. This started back in 1960. Two further major milestones are the
successful participation in the European large E- and F-class gas turbine IGCC projects of
Buggenum / Nuon (The Netherlands) and Puertollano / ELCOGAS (Spain).
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Fig. 10: Siemens syngas reference list
Regular benefit evaluation and availability improvements have resulted in syngas plants with
proven commercial potential. The Buggenum plant – owned and operated by Nuon – has
achieved a yearly operating run on syngas of between 6,000 and 7,000 operating hours [7].
This is the same as for operation of a base-load standard fuel plant.
Syngas provides increased flexibility for the use of available energy sources. Syngas can be
produced from a wide variety of coal grades, but also from biomass, petroleum coke and
refinery residues. Purified synthetic gas not only increases the flexibility of the useable
energy sources, it also enables use of these energy sources in gas turbines targeting
electricity, heat production and CO2 sequestration & storage.
The difference – as defined by Siemens – between low-BTU and syngas is that a low-BTU
gas does not have any hydrogen (H2) fraction beyond the standard values. By contrast, syngas
has one major component and that is hydrogen (H2). H2 is provided by the gasification
process and is a function of feedstock and syngas conditioning upstream the gas turbine. The
nature of H2 is that it has the highest reactivity and flame speed relative to all other
combustible gases. This fact must be considered in design of the combustion system and in
the associated high pressure combustion test campaigns.
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To match the reactivity of the syngas to the combustion kinetics of standard fuels, the syngas
is blended with nitrogen (a byproduct from the air separation unit), CO2 and/or steam as inert
fractions.
Syngas has a very low heating value of between 4 and 12 MJ/kg, with the result that fuel flow
– at constant thermal heat input – is higher than for standard fuels. This effect has to be taken
in account with regard to the surge limit for the gas turbine compressor and its design. Due to
the low LHV, the fuel gas system has to be modified based on proven design technology to
accommodate the resulting large quantities of fuel.
This higher "effort" in handling the increased fuel flow yields an increase in power output
and efficiency.
Gas turbines operating on syngas produce very low NOX emissions and are nearly as clean as
pure natural gas power plants. [1]
The gas turbine is started in secondary fuel mode (natural gas or fuel oil) and run up to a
defined minimum output before changing over to syngas operation. The gas turbine is also
returned to the secondary fuel prior to shutdown.
The plant concept for a gas turbine operated with syngas can be an IGCC plant. Here
Siemens has gained experience with both fully integrated and non-integrated plants (see
Figure 11 and Figure 12).
In one concept, air is extracted from the compressor end of the gas turbine and separated into
its primary components oxygen and nitrogen. The oxygen is used to generate the syngas. The
syngas, which has been purified as required to meet the specifications of the gas turbine, is
saturated with water in a saturator to reduce NOX and is subsequently superheated. The other
major concept is the non-integrated (see Figure 11 (1)) one. This provides the highest
flexibility and the GT can operate on secondary fuel independently of the gasifier e.g. during
gasifier maintenance.
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Fig. 11: Power plant configuration, non-integrated, partial and full air side integration
Figure 12 shows the process parameters and the gas composition fractions for various
successful applications using Siemens gas turbine technology. [2]
Fig. 11: Syngas applications with Siemens gas turbines.
2.2.2
Application of syngas
Due to the significant difference in the amount of fuel compared to natural gas, Siemens
developed a special syngas burner (see Fig. 12) in the early 90s of the last century. It has been
successfully used in a number of syngas plants since the mid 90s. This burner includes a
second duct in the gas burner for the secondary fuel required for startup and shutdown.
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Fig. 12: Siemens syngas burner.
Because the fuel composition can vary widely, the fuel will be analyzed and assessed
individually for each application. This analysis is essential to obtain information on necessary
modifications to combustion and auxiliary systems. High pressure combustion tests are
scheduled, if necessary. Analysis here also serves to define thermodynamic performance and
emissions.
In addition to these project-specific modifications, it must be remembered that syngasspecific supply systems designed for gas turbine operation are required in addition to the
usual fuel supply systems: air separation unit, syngas generation/purification, saturator for
NOX water, syngas superheater, possibly also a mixer for the N2 feed and inerting systems.
Siemens has also further developed its very successful E-class gas turbine - the SGT5-2000E
- for use in syngas applications. This engine has been designed and validated to enable it to
meet with the technical and commercial requirements for such gas turbine applications. It is
subject to an ongoing development process to provide even higher power output with
improved heat rate and increased fuel flexibility for all kinds of air-side integration.
2.3
Crude oil
2.3.1. Overview & features of crude oil
Crude oil is a naturally-occurring substance found in certain rock formations in the ground. It
is a dark, sticky liquid that is classified as a hydrocarbon. This means it is a compound
containing molecules made of carbon and hydrogen atoms, with or without non-metallic
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elements such as oxygen and sulfur. Crude oil varies greatly, both in its potential use and
composition. It can be a straw-colored liquid or a tar-black solid. Red, green and brown
coloring is not uncommon. Natural gas derivatives after fuel treatment, e.g. by refining are
common fuels.
Although various types of those hydrocarbons are based on petroleum, they differ in their
configurations. The carbon atoms may be linked in a ring or a chain, each with a full or
partial complement of hydrogen atoms. The number of carbon atoms determines the oil's
relative "weight" or density. Gases generally have one to four carbon atoms, while heavy oils
and waxes may have fifty, and asphalts hundreds. Some hydrocarbons combine easily with
other materials, and some resist such bonding.
A key fact for refineries separating the different components of crude oil by weight and
boiling point is that hydrocarbons also differ in their boiling temperatures. The lightest
hydrocarbons, gases, boil below atmospheric temperature. Crude oil components used to
make gasoline boil in the range of 12°C to 200°C. For comparison, those components used
for jet fuel boil in the range of 150°C to 300°C and those for diesel at about 350°C. [4]
2.3.2
Application of crude oil
The crude oil liquid fuel supply system features a similar configuration to that previously
described in the section on heavy fuel oil. The functional and mechanical design of this crude
oil fuel supply system has to consider the special requirements that apply for heated crude oil
with respect to degasification as well as density, viscosity and flash point in accordance with
applicable safety regulations.
For safety reasons:
An additional startup and shutdown liquid fuel oil is required to avoid any residues of
crude oil in the system during standstill. The shutdown program includes a subsequent
flushing procedure with liquid fuel oil in the event of a gas turbine trip. In the event of a
grid blackout, manual draining into a dedicated, closed system and flushing with nitrogen
is mandatory. To rule out blocked drain pipes, installation of an emergency diesel to
supply the unit with sufficient electrical energy in case of grid blackout is required scope.
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This will keep the heat tracing in operation and enables the forwarding pump system to
perform an emergency purge with liquid fuel oil.
The entire gas turbine, or at least specific areas of the turbine as well as the fuel skids
have to be enclosed and ventilated, and electrical equipment has to fulfill ex-protection
requirements.
Double stem seal systems for the liquid fuel valves consisting of metal bellows made of
high grade steel with additional self-resetting safety stuffing box and a possibility for
connecting a leakage display must be used. Specially sealed bearings for the liquid fuel
pumps must be used as well.
Due to similarities between crude oil and heavy fuel oil applications, Siemens' experience
with various heavy fuel oil units can be taken into account for crude oil applications.
3
Developments for tomorrow
The increasing demand for energy and the rising prices for standard fuels require further
improvements in fuel flexibility for use in gas turbines.
Key to the future success of our customers is accelerated development of the combustion
systems. The major contributor here is high pressure combustion testing prior to delivery of
the gas turbine to the customer.
If Siemens paints an optimistic picture for the various fuel applications in its fleet, this is due
to its many years of experience in this field. The customer requirements for potential
modifications are as different as their specific parameters, however. Thus a detailed,
customer-specific study is generally required as early as the bid negotiation / implementation
phase.
Siemens is actively pursuing developments in the E- and F-class to make them more and
more suitable – even beyond the current fleet requirements - for non-conventional fuels, with
focus here on the combustion of syngas and/or low-BTU gases.
Siemens is also targeting making all fuel flexibility measures retrofittable to the service fleet.
Copyright © Siemens AG 2008. All rights reserved.
Siemens is focusing all these actions on benefit for our customers throughout the world and
on protection of the environment.
4
Literature
[1]
B. Becker, B. Prade, M. Karg: Siemens gas turbine featuring low caloric gas
applications, Power-Gen 2004, Asia.
[2]
H. Hermsmeyer: Erfahrungen mit Synthesegas, LowBTU und variablen
Gaszusammensetzungen in Siemens Gasturbinen, November 2006, VDI Tagung
Leverkusen, Germany.
[3]
E. Deuker, R. Waldinger, H.U. Rauh, F. Schade: Multifuel Concept of the Siemens
3A
Gas turbine Series, ASME, Turbo EXPO, June 2001, New Orleans, Louisiana, USA.
[4]
Chevron Products Company, San Ramon, CA. www.chevron.com ©2007.
[5]
M. Alf: Effizienzsteigerung von thermischen Kraftwerken, WEC-Workshop,
November
[6]
2006, Vienna, Austria.
V. Poloczek, a.o., Siemens AG Energy Sector; Fuel Applications in Modern Gas
Turbines; PowerGen Middle east 2008
[7]
R. de Kler, Nuon, Gasification Technologies 2007 15 Oct 2007, San Francisco,
Presentation
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