Figure 3 3 1 Generic IGCC Flow Diagram

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Figure 3 3 1 Generic IGCC Flow Diagram Powered By Docstoc
					                                                             Steam                  Heat Recovery
                                                                                    Steam
                                                                                    Generator
                            Gas


                Gasifier      Fuel         Fuel                  Gas
 Coal
                              Gas          Gas                   Cleaning
                                           Cooler
              1600oC              1000oC             250oC
              30bar               30 bar             28bar                     Gas
                                                                              Turbine                Steam
                                                                                                    Turbine




Figure 3.3.1 Generic IGCC Flow Diagram



                                                                                        HEAT RECOVERY
                                                         STEAM                          STEAM
                                                                                        GENERATOR


             FUEL GAS
                                     FUEL GAS                 HOT GAS
                                     COOLER                   CLEANING

                            1000oC           400-600oC                      400-600oC
                             25bar             23bar                          21bar
              G
              A
              S
 1000oC       I
 25bar        F                                                                 GAS
              I                                                               TURBINE
              E
              R                                             STEAM
                                        CFBC

                                                          SUPERHEATED
                     CHAR                                    STEAM
            COAL
              +
             AIR
              +
          DOLOMITE                                                            STEAM
                                                                             TURBINE




Figure 3.3.2 Air Blown Gasification Cycle Flow Diagram.




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Figure 3.3.3   Oxidation/Sulphidation Damage in Gasification Environments




Figure 3.3.4   Oxidation/Sulphidation Corrosion Model for Type 310SS



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3.4      Gas Turbines


3.4.1    Gas Turbine Market Trends

The success of the gas turbine power generation industry in the UK is dependent upon satisfying
the demands of a range of customers and operators and by optimising the complex technical
challenges arising from a number of market and legislative factors:

•     Capital Costs
•     Operational Costs
•     Efficiency
•     Power Output
•     Fuels
•     Emissions

The emphasis placed on each of these factors by any organisation within the power generation
industry depends on the particular role played within the market, i.e., manufacturer, supplier,
operator etc1. The following sections aim to describe the technologies and commercial and
developmental drivers for land based gas turbine materials and components. The market trends
and current state of the art and future prospects for component design and operating conditions
(e.g., pressure ratios, turbine inlet temperatures etc) are described. Figure 3.4.1 shows an
ALSTOM Power large IGT engine and illustrates the wide range of materials used to
manufacture the components for these engines; a range of ferrous and nickel-based alloys are
used, often having oxidation, corrosion and thermal protection coatings.

The manufacturing facilities needed to produce these components are often complex requiring a
wide range of engineering and materials skills to ensure safe, reliable operation of the engines as
effective power generation units. A full description of the manufacturing routes is beyond the
scope of this report, however, each stage of manufacture: raw material production and refining,
melting technology, ingot or near-net shape casting, isothermal forging and machining, joining
and coatings applications, forms a key part of the component production process. The report
does provide, however, an assessment of the conventional and advanced materials systems
needed to meet the market and operational challenges. Issues associated with materials supply
and production, as well as component design and manufacture (e.g., casting, fabrication and
welding) are addressed, where relevant. Other issues to be covered are so-called second order
issues such as plant operation effects, co-firing, fuel flexibility, repair and overhaul,
maintenance, component life assessment and life extension. A review of the current status for
materials technologies enables the materials development requirements to be identified and,
thereby, a strategy for resolving these issues to be established. These have been summarised in
the concluding sections.

Gas turbines feature as key components of the most efficient forms of advanced power
generation technology available. The high versatility and flexibility, in terms of the possible
core diameters and number of stages, enables gas turbines to be used as a means of generating
electrical energy using operational cycles such as conventional simple cycle, combined cycle
and combined heat and power generation systems. A range of fuels are used, including natural
gas, synthetic gas produced from coal or other feedstocks, biomass or liquid fuels. More
complex cycles that incorporate hardware such as intercoolers, recuperators and heat exchangers
and reformers are being developed that aim to provide improvements in thermal efficiency, but
consequently lead to more complex running conditions for core components and more complex
plant design and operation. Other issues include the use of hydrogen fuel and storage, CO2
recycling and sequestration and use of fuel cells and catalytic combustion systems. To exploit
integrated gasification combined cycles (IGCC), much work has concentrated on the
development of air staged gas turbine combustion using hybrid cycles. Air blown gasification
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(ABGC) offers the potential for cleaner coal technology that benefits from increases in gas
turbine efficiency and super critical steam cycle developments to produce lower emissions.
However, the properties of lower calorific gas (LCG) fuels produced by hybrid cycles present
several problems in terms of combustion systems such as:

•   increased fuel mass flow for a given heat input requirement;
•   presence of fuel born nitrogen (FBN) leading to higher NOx emissions;
•   increased fuel temperature due to exothermic gasification processes.

These factors require significant modifications to the combustion system to maintain
performance.

The future global energy demand for power generation has been projected to increase from
approximately 13,000 TWh in 1995 to 20,000 TWh in 2010 and up to 25,000 TWh by 2020,
which represents an annual growth rate of 2.7% 2,3. Natural gas is still expected to be the
preferred option fuel to meet this demand where it is available, due to a number of
technological, economic and environmental advantages. Natural gas combined cycles provide
cost effective power generating efficiencies approaching 60%, with low NOx, SOx and CO2
emissions compared with coal fired systems. That said, over the next 20 years the major fuel
type used globally will be coal, providing approximately 40% of the electricity demand. The
importance of coal within the EU is expected to decline; however, electricity generation in
developing countries having large reserves of coal is expected to triple in size by 20203.

The key factors for coal can be summarised as follows:
• widely available throughout the globe with significant reserves (estimated at ~200 years);
• anticipated high demand from countries that have indigenous reserves;
• natural gas is less widely available;
• socio-economic factors that maintain infrastructure and promote use of indigenous coal
   reserves;
• helps to maintain diverse fuel supply mix.

In addition to the demand for coal-fired systems, there is an increasing need for renewable
energy resources such as solar, wind, geothermal and biomass. The combined fuel-mix share for
these technologies is expected to increase to 2.3%, a trend that has strong political backing.
Globally hydropower generation accounts for 18% of the fuel share mix. The EC has
established a commitment to a 12% contribution from renewable resources (including
hydropower) to the European electricity market. To meet these demands continued development
of coal fired power systems is required to enable a balanced, flexible approach to the power
generation supply mix to be taken.

For the foreseeable future, regardless of the fuel type used, all of the major gas turbine engine
manufacturers are looking to target improvements in efficiency and lower emissions. Those
engines operating directly in coal gasification plant will need to meet the challenges made by
the performance capabilities of natural gas fired turbines as well as the technical material
challenges brought about by the use of aggressive fuel gas. Efficiency and emissions targets will
be achieved by raising the pressure ratios and turbine inlet temperatures leading to more arduous
service conditions on critical turbine components in the hot gas path. This must be achieved
without detriment to the reliability, availability and maintainability (RAM) of the engines in
operation. In terms of power output, the growth of the aero-derivative engines such as RR Trent
from 50 MW is inevitable and for utility scale engines, growth to power outputs in excess of
350 MW units is likely to occur. Generally, the material challenges in coal-fired plant will be
greater than those utilising premium fuels. This is due to the additional problems of gaseous
reducing environments, high temperature corrosion from fuel contaminants (sulphur, alkali
metals, vanadium), low temperature downtime corrosion during turbine shutdown, and erosion
by gas-borne particulates. For current generation IGCC systems with wet scrubbing for gas
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cleaning, virtually all the contaminants of concern are removed and so few gas turbine
erosion/corrosion problems should be anticipated. However, the trend towards the use of hot
gas cleaning to improve cycle efficiencies and reduce capital costs means that efforts must be
directed to produce improved blade materials and coatings (corrosion resistant and thermal
barriers) to ensure that adequate blade lives and RAM requirements are achieved.


3.4.2   Gas Turbine Design, Operation and Materials

The design and manufacture of gas turbines for power generation systems is specified/regulated
by the American Petroleum Institute Standard 616 (small to intermediate engines) and the
regulations/conditions imposed by the operators insurance companies. The following sections
describe the current and anticipated component design and operating conditions for the core
stages of small to intermediate and larger IGT engines and aim to identify the technical
challenges and requirements that need to be addressed for each application.

Compressor

The component design issues facing the compressor section are largely associated with the need
to enhance existing designs and establish the framework for next generation systems. The
challenges to be met are to provide improved cycle efficiency, operability and reduced costs by
optimising the work done stage-by-stage. These targets are aimed at providing improved
operating costs and better control of the pressure ratio and mass flow characteristics through the
compressor during both high and low ambient running conditions, as well as during start up and
turndown. In addition, improved reliability and availability are key deliverables currently being
addressed. Increasingly advanced manufacturing methods are being introduced to promote cost
reduction and improved integrity. For example, welded structures are being considered to
replace bolted joints, by means of friction and/or electron beam (EB) welding methods.

Future developments are aimed at increasing pressure ratios from the current 15:1 level to a
maximum of 30:1, whilst continuing to target efficiency and surge margin gains. The need to
maintain compressor performance and integrity through life, whilst reducing parts costs and the
use of more effective manufacturing processes is prevalent, as is the need to achieve operational
lifetimes in excess of 50,000 hours. The development of more complex cycle configurations is
also being addressed to meet the demands for inter-cooled systems, for example. Many of these
targets are dependent upon improved design and aerothermal analysis methods in conjunction
with test and validation procedures.

For small to intermediate IGT compressors (see Figure 3.4.2), the temperature loadings
experienced currently range from –50 to less than 500ºC, and usually do not present any
significant challenges to the materials engineer. The continued use of low alloy and ferritic
stainless steels has proved to be adequate and this situation is likely to continue unless
significant increases in compressor temperatures are needed due to much higher pressure ratios
and rotor speeds. In such a situation it has been assumed that aeroderivative technology such as
titanium alloys, nickel alloys and composites will be employed. This would, however, present a
significant increase in cost and manufacturing complexity (forgings, machining, joining,
component lifing) as well as operational difficulties (component handling, overhaul, repair,
cleaning) and may introduce additional problems associated with thermal mismatch and fretting
fatigue from adjoining ferritic alloys. A number of materials-related issues would need to be
addressed.

Consideration has also been given towards lightweight materials such as aluminium matrix
composites, polymer composite blading and vanes, as well as intermetallic TiAl-based alloys to
provide reduced rotor and overall engine mass, and lower disc stresses to enable higher
rotational speeds. In addition, design and materials concepts have evaluated the application of

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integrally bladed discs (bliscs) based on steel or titanium alloy technology using friction
welding. Some issues associated with rotor corrosion are largely operator dependent, being
influenced by compressor washing and cleaning practices and are addressed by protective
coatings. Likewise commercially available abradable tip sealing coatings are used to provide
and maintain efficiency and these currently present little technical risk.

For large utility power generation engines, however, targeting >60% efficiency and with >500
MW CCGT performance, the temperature and strength limitations of the rotor steels used
currently are limiting the successful achievement of these performance capabilities. Within the
EU, the development and demonstration of high nitrogen, nano-precipitate strengthened steels
for high pressure compressor disc applications (see Figure 3.4.3), offering equivalent strength
and temperature capabilities to some nickel alloys with much reduced cost, is crucial in
achieving these goals. Application of these high strength creep resistant steels necessitates the
development of improved large scale melting (up to 100 tons) and forging capabilities (up to 18
tons) and the development of suitable welding technologies, NDT methods for large-scale rotors
and validated life-assessment and risk-analysis methods. Successful development of this
technology would negate the need to introduce much more expensive (i.e., by a factor of 5)
nickel alloy technologies currently being targeted in the USA.

Combustor

The design objectives for combustor technologies aim to satisfy the commercial requirements
by providing reduced costs, reduced emissions (CO2 and NOx), improved turndown operation,
increased lifetime and to meet the demands for new innovative cycles. Combustion engineers
rely on developing a clear understanding of the combustion process and the influence of factors
such as turbulent flow, thermo-acoustic loadings and mechanical integrity. The aim is to enable
more rapid extrapolation of combustion technologies for new markets as well as rapid resolution
of field issues. A range of testing and modelling methods have been developed to help optimise
the combustor design and operation. Water flow testing, spray tests, engine and rig test data, as
well as field experience are combined with computer modelling methods to provide an
integrated combustor development process. Figure 3.4.4 shows a series of IGT combustor cans
developed for a range of intermediate sized engines. Much of the technology is based on
providing enhancements to lean burn, dry low emission (DLE) combustion technology;
however, significant effort has targeted developments for the use of low calorific value (LCV)
fuels from gasification using ABGC and catalytic pre-treatment systems using hybrid
combustion configurations. In this field significant effort is needed in the development of cost-
effective, highly active and stable catalytic materials (e.g., those based on PdO-Pd systems) that
are self-regulating with respect to operating temperature. Resistance to sintering and
degradation of the catalytic material are key issues, as is cost reduction (e.g., development of
doped-alumina based materials) and operating life.

Currently, the temperature loadings experienced range from 1525 to 1650K, depending on the
engine size and duty cycle. The targets set are for dual-fuel operation producing less than 5 ppm
NOx for gas and less than 35 ppm for oil fuel. Other LCV or LCG-type fuels present a range of
complex emission related problems due to intrinsic FBN levels. The target lifetime under these
conditions is in excess of 24,000 hours. Future developments aim to reduce NOx emissions
further to meet anticipated environmental legislation and customer demands for cleaner running
by optimising the distribution of fuel during firing and catalytic combustion systems. The
significance of this requirement is to place a limit on the anticipated future turbine entry
temperature (TET) levels, placing more emphasis on controlling peak flame temperatures within
the combustor. This should also provide more air for cooling the combustor liner; however, as a
consequence, other components may be required to run hotter as the demand for combustion
flame temperature control increases.



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That said, the combustor experiences the highest gas temperatures and is subject to a
combination of loadings; pressure variations in the combustion process can lead to high cycle
fatigue, whilst start-up and shut-down can cause thermal fatigue in designs where significant
restraints are introduced. Temperature differentials across the combustor walls, leading to
localised hot-spots, emphasises the requirements for endurance under creep and thermal fatigue.
The materials used presently are generally wrought, sheet-formed nickel-based superalloys,
such as Hastalloy X, Nimonic 75 or Haynes 230. These provide excellent thermomechanical
fatigue, creep and oxidation resistance for static parts and are formable in fairly complex shapes
such as combustor barrels and transition ducts. Of equal importance is their weldability,
enabling design flexibility and the potential for successive repair and overhaul operations,
which is crucial to reducing life-cycle costs. The high thermal loadings imposed often mean that
large portions of the combustor hardware need to be protected using thermal barrier coatings.

Of increasing importance is the ability to calculate more accurately the lifetime of combustor
hardware during service loadings. In particular, the development of damage and cracks
associated with localised hot-spots that suffer creep during the high temperature part of the
thermomechanical fatigue (TMF) cycle, which is reversed by plastic deformation during the
engine shut-down, needs to be addressed. This form of cycle often develops open hysteresis
loops that lead to materials ratcheting, damage accumulation and shortened lifetimes.
Programmes of work have addressed the development of more sophisticated constitutive models
for combustors capable of simulating the materials behaviour during TMF loading experienced
during service. Model validation and incorporation within non-linear, finite element stress
analysis routines, in conjunction with more accurate damage accumulation models, is crucial to
the development of improved design and lifing methods.

Materials technology acquisition programmes for future small to intermediate engine combustor
designs are aimed at the replacement of conventional wrought nickel-based products with either
oxide dispersion strengthened (ODS) metallic systems or ceramic matrix composites (CMCs).
These programmes are primarily aimed at addressing the limitation in temperature capability
and coating compatibility of the conventional alloys used currently. Candidate ODS and CMC
materials have been identified and demonstration hardware manufactured and, in the case of
CMC components, engine tested. However, there are limitations to these technologies that need
to be addressed. For example, both joining methods (based on for example laser welding or
brazing) and coating systems, including TBCs, need to be developed for ODS combustion
hardware. These materials have been identified as candidates for efficient, high temperature heat
exchangers for a range of externally fired combustion (EFC) systems that separate the turbine
working fluid from the aggressive combustion gases generated by poor quality fuels. This limits
the damage incurred by hot section hardware during engine running and enables the use of a
range of LCV and biomass fuels combined with CHP recovery systems.

A programme of work has been underway to establish CMC combustor technology as a viable
alternative to metallic systems. Previous work has developed the manufacturing and design
capabilities (see Figure 3.4.5), however, much work remains to be done to improve the lifetime
prediction methods and to develop coatings to provide thermal and environmental protection of
the combustor liner. At temperatures in excess of 950°C, the CMC fibres in SiC/SiC-based
composites degrade rapidly as a consequence of oxidation leading to poor structural integrity of
the liner during operation. The programme objectives are to develop thermal protection systems
(TPS) for these materials that act as a thermal and environmental barrier for the substrate and
establish CMC combustor technology running at up to 1600K. Also, there is a need to identify
alternative candidate materials based on oxide-oxide CMCs that do not suffer such
environmental degradation and potentially offer significant cost savings over the SiC-based
materials.

Current TBC technology for combustor applications is based exclusively on multi-layered
systems comprising of an MCrAlY bondcoat and a ceramic topcoat applied using plasma spray
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deposition techniques. Application of this technology generally aims to limit peak metal
temperatures to 900 to 950°C. Future developments are directed at applying thicker TBCs to
enable higher flame temperatures and/or reduce metal temperatures further. Other programmes
are aimed at increasing the phase stability and resistance to sintering of the ceramic topcoat at
temperatures above 1250°C.

Finally, the use of coal gasification cycles may lead to much higher particulate loadings than for
other fuels. The development of high temperature erosion and corrosion resistant coatings and
substrate materials, as well as improved gas cleaning facilities will be required. The increased
operating temperatures and corrosive/erosive fuel gas will require the further development of
coating technologies. Other issues currently being addressed for future combustor development
are advanced technologies, such as steam injection and power augmentation.

Turbine

The following section reviews the current state of turbine technology and identifies the
materials challenges for the relevant components. Each of the turbine sections present a range of
materials and design issues for current and future engines that are dependent on the engine size,
operation and duty cycle imposed. The design objectives are to develop more efficient running
and improved performance for the turbine, whilst reducing life cycle costs and increasing
reliability. New co-generative, advanced cycles based on steam injection methods as well as
more novel cooling configurations for the first stage vanes and blades, such as radial and axial
air/steam injection, are being evaluated. In addition to changes to the working fluid, increased
efficiency has been targeted by optimising the design of harder working aerofoils with increased
temperature capability and improved control of losses from the hot gas path and cooling air
from secondary air systems. The development and improvement of aerothermal design methods
is key to these objectives and significant improvement in the effectiveness of cooling schemes
in blades and vanes has provided marked benefits to the operational characteristics of the engine
fleet. Both internal and externally cooled vanes and blades have been developed and
successfully introduced to market. Figure 3.4.6 shows a typical four-stage turbine for an
ALSTOM Power IGT engine, which is comprised of a significant number of single crystal and
directionally solidified investment cast parts.

The need to supply more effective cooling to the turbine discs is also a key requirement for the
design engineers. This work compliments a number of materials and component design
activities aimed at increasing the temperature and loading carrying capabilities of the turbine
disc designs. Also of increasing relevance is the improvement and maintenance of turbine
efficiency due to high performance sealing systems. These are largely based on honeycomb plus
backing plate technologies in conjunction with abradeable coatings and effective tip-sealing on
the corresponding rotor stage.

Turbine Blading

Turbine blades are subjected to significant rotational and gas bending stresses at extremely high
temperatures, as well as severe thermomechanical loading cycles as a consequence of normal
start-up and shut-down operation and unexpected trips. The TET for a number of engines is in
excess of 1650K, with base metal temperatures ranging from 850 to 1050+°C, depending on the
specific engine type, the cooling efficiency and operation. The target lifetime under these
conditions is dependent on engine type and duty cycle, but can be in excess of 24,000 to 50,000
operating hours (OH). The blades pass through the wake of the combustor and nozzles and are
subject to frequency excitations, which can lead to high cycle fatigue failure. The high-pressure
stages are cooled to withstand the hot gas temperatures and, depending on the type of fuel,
severe corrosion and erosion of the blade structure is limited by the use of protective coatings.
For many years the primary consideration in the design of blades has been to avoid the
possibility of creep failure due to the combination of high stresses, temperatures and the
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expected length of running time for land–based engines. The development of alloys to improve
mechanical properties is a continuing need and component reliability is the prime commercial
driver with more stable, long-term mechanical properties and lower cost (castability, production
yields) being identified as the key objectives.

To meet the requirements for increased turbine temperatures, more advanced materials have
been introduced into the turbine sections of high performance, power generation units (Figure
3.4.6). For vanes and blades there has been a gradual move away from conventionally cast
nickel-based superalloys, such as IN939, IN738 and IN792, towards directionally solidified
(DS) alloys such as Mar-M247, IN6203DS and CM186LCDS. The introduction of these alloys,
manufactured using near-net shape investment casting has provided significant benefits in terms
of much improved creep and thermal fatigue properties. Further significant benefits have been
gained by the use of single crystal (SC) technology using alloys such as CM186LCSX and
CMSX-4. A number of issues have, however, still to be resolved. The increased cost of
manufacture, due to high alloying levels and parts rejection, needs to be carefully controlled by
the use of revert materials and control of the casting conditions, and offset against improved
component lifetimes and more efficient running by enabling higher TET levels to be achieved.
To achieve increased creep strength, successively higher levels of alloying additions (Al, Ti, Ta,
Re, W) have been used to increase the level of precipitate and substitutional strengthening
available at high temperatures. These alloys are extremely creep resistant and have been the key
to the success of the aero gas turbine industry and increasingly the land-based sector. However,
as the level of alloying has increased, the chromium (Cr) additions made have had to reduce
significantly to offset an increased phase instability problem, wherein deleterious phases
precipitate out of solution after long-term thermal exposure. These phases lead to limited
ductility and reduced strength levels. The consequence of having to reduce the Cr level is to
reduce significantly the corrosion resistance of the alloys. This has necessitated the development
of a series of protective coating systems to meet the range of fuels types used by various
operators. These coatings are applied to provide increased component lifetimes, but they often
demonstrate low strain to failure properties that can impact upon the thermomechanical fatigue
endurance.

In addition to these considerations, the industrial gas turbine manufacturer must address serious
issues associated with the long-term stability and mechanical properties of the DS and SC
materials used, as well as the manufacture of significant numbers of large components, some up
to 0.5 metre in length. The manufacturing methods (lost wax investment casting) and materials
properties (peak temperature creep strength capability) for the majority of the alloys
commercially available have been designed to meet the needs of the aero-engine manufacturers.
The component scales and engine duty cycles often differ significantly from those in the
industrial market. Figure 3.4.7 shows a schematic illustration of the different temperature
loadings to which the first stage blade is exposed for a typical aero and industrial cycle. The
perception has been that the aero-engine suffers more arduous running conditions. However,
although the aero-engine may see higher peak temperature loadings for short periods during
take-off, and may also be exposed to many more cycles, it is clear that the overall running
conditions for the industrial turbine can be much more arduous in terms of exposure to long
times at high temperatures.

That said, many of the smaller engines manufactured by the industrial GT business have been
able to draw across much of the casting, manufacturing and materials technologies developed
for the aero-engine business. This has often satisfied the needs for higher operating temperatures
without too much difficulty, apart from problems associated with dirty, more corrosive fuels.
Currently the challenges are to improve casting quality and yield to reduce costs and improve
reliability further. Also, more efficient internal and surface film-cooling techniques are needed
to improve component durability and reduce cooling airflow requirements.



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The same cannot be said for the large, utility power generation engines. Large single crystal
castings are extremely expensive and difficult to produce without the occurrence of
microstructural defects, such as freckles, high angle grain boundaries and coarse dendritic
structures due to the low thermal gradients generated in the larger castings. To achieve high
quality castings, modifications have been made to the conventional Bridgeman casting
technique as illustrated in Figure 3.4.8. Some manufacturers have introduced liquid metal
cooling (tin or aluminium) to quench the casting as it emerges from the furnace; however, this
method can lead to contamination or problems with volatilisation of the bath metal.
Alternatively, gas convection cooling may be used to enable increased withdrawal rates and
produce a refined, defect-free microstructure. Introduction of this technology has been
dependent on the development of advanced computer-based fluid-flow and casting simulation
software, which has enabled the casting process to be evaluated prior to manufacture of large-
scale foundry facilities. Further detailed optimisation of the casting process is possible using
finite element-based modelling methods. The further development of design by modelling
methods (casting through to long-term mechanical properties) is seen as the key to achieving
optimum utilisation and cost reduction targets. As an alternative to casting large blades, several
manufacturers have developed so-called transient liquid-phase bonding methods (similar to
brazing) to construct large SC components from smaller cast segments.

The development of IGT-specific turbine blade alloys continues to be a difficult problem to
resolve. Much dependence has been placed by the land-based sector on the transfer of advanced
technologies from the aero sector and this has not always provided the necessary solutions. The
key issues associated with this dependence are as follows:

•   Development of a succession of alloys with increasingly lower corrosion resistance, despite
    increasing requirements for the use of differing poor quality fuels and a range of running
    conditions to satisfy the power generation market requirements.
•   Limited castability of large-scale components due to recrystallisation and microstructural
    defects such as freckles, large angle grain boundaries and coarse dendritic structures leading
    to reduced property levels.
•   Over emphasis on high stress-high temperature creep strength to satisfy certification and
    type-test approval for take-off conditions for aero engines.

Figure 3.4.9 illustrates the diverging requirements of the aero and industrial sectors for SC blade
alloy technology. In recent years both improved creep strength and long-term phase stability
were satisfied by the progression from conventional cast to DS and SC systems. However, as
the need for increased strength for aero-engine applications took precedence with alloy
developers/component suppliers, there has been a deleterious effect on the stability of the alloys
during long term exposure. This has been attributed to the high precipitate volume fractions
(>70%) needed to achieve high temperature creep strength.

Efforts have been made to address these issues with the development of a number of IGT
specific alloys having improved castability, higher corrosion resistance and reduced heat
treatment times. Alloys such as SC16, MK4, CMSX-11 and SCA425 have been developed with
varying degrees of success. Further work is needed in this field, however, as it will remain a key
issue for future IGT turbine designers, impacting directly on the performance and RAM
capabilities that can be offered. The industrial GT alloy development needs to maintain strength
levels equivalent to current second generation SC alloys, whilst increasing long-term stability.
To achieve these goals it is proposed that it will be necessary to decrease the volume fraction of
the strengthening phase to <60%, whilst increasing the substitutional strengthening additions
without promoting the precipitation of deleterious TCP phases. Improved oxidation and
corrosion resistance is also needed by the introduction of cleaner manufacturing (reduced
sulphur and alloy doping) and increased Cr additions. To improve castability, additions of
elements such as carbon have been made to improve fluidity and provide grain boundary
strengthening, should secondary grains be nucleated, to reduce the susceptibility to casting
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defects. In addition, the relative solid-to-fluid density needs to be balanced to control freckle
and sub-grain formation in large castings.

The increased use of high-cost turbine hardware and the need to provide more accurate lifetime
predictions has led to the development of improved finite element stress analysis and materials
modelling techniques. A number of anisotropic creep and plasticity models have been
developed to simulate the high temperature deformation and enable more accurate simulations
of the component shakedown or stress evolution during service exposure. These methods are
capable of providing improved fatigue life calculations compared with conventional isotropic
models; however, the complexity of the numerical calculations and the computer run-times
necessary have, to a large extent, restricted the wider use of these life assessment methods.
Further work is vital to improve the capabilities of the models developed, as well as to validate
and demonstrate the benefits in terms of more accurate stress analysis calculations and, thereby,
more effective materials and component utilisation.

Commensurate with these considerations is the development of component repair and overhaul
technologies in conjunction with remnant life assessment methods to satisfy the customers’
demands for reduced life cycle costs. In addition to re-establishing component profile, this work
includes the potential for HIP rejuvenation and heat treatment of hot-section parts to recover
used creep life. Figure 3.4.10 shows an example of a laser-based blade tip repair process which
deposits a SC layer epitaxially onto the underlying substrate to maintain the single crystal
character of the blade tip metal. Equally of importance is the development of braze repair
techniques for conventional, DS and SC blades. These methods provide increased overall
component lifetimes and a recovery of the turbine stage efficiency by reduction of losses due to
poor sealing. In conjunction with NDT and metallographic examination of service-exposed
parts, repair and overhaul methods are used to extend the lifetime to several successive overhaul
periods. For certain large-scale components, HIP and re-solution and ageing treatments can
provide improved component utilisation and reduced costs.

For the later turbine stages, such as the low pressure or power turbine, extensive use is made of
conventionally cast alloys such IN738LC and MarM247 depending on the particular
temperature loadings and corrosive environment to be encountered. Recent studies have
assessed the potential application of titanium aluminide (TiAl) alloys to meet the needs for
harder working, high speed power turbines to provide significant improvements in efficiency
(>3%). Application of TiAl blades (see Figure 3.4.11) would provide much reduced disc
stresses, but significant difficulties remain to be resolved that are associated with near-net shape
casting, machining and life assessment.

Turbine Discs

The main functions for a turbine disc are to locate the rotor blades within the hot gas path and to
transmit the power generated to the driveshaft. To avoid excessive wear, vibration and poor
efficiency this must be achieved with great accuracy, whilst withstanding the thermal,
vibrational and centrifugal stresses imposed during operation, as well as axial loadings arising
from the blade set. Under steady-state conditions, turbine disc temperatures can vary from
approximately 450°C in the cob to in excess of 650°C close to the rim with a requirement for
>50,000 hrs operating life. These temperature loadings are set to increase further across the disc
as the demand for improved efficiencies continues.

Creep and low cycle fatigue resistance are the principal properties controlling turbine disc life
and to meet the operational parameters requires high integrity advanced materials having a
balance of key properties:
• high stiffness and tensile strength to ensure accurate blade location and resistance to over-
   speed burst failure;


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• a combination of high fatigue strength and resistance to crack propagation to prevent crack
  initiation and subsequent growth during repeated engine cycling;
• creep strength to avoid distortion and growth at high temperature regions of the disc;
• resistance to oxidation and corrosion attack and the ability to withstand fretting damage at
  mechanical fixings.

As the tensile strength of disc alloys increases, a commensurate decrease in crack growth
resistance and damage tolerance has been found that has serious implications for the lifing
methods used. Some of the properties can be attributed to differences in microstructure;
however there also appear to be fundamental differences between the grain boundary oxidation
behaviour that determines the inherent crack growth resistance of the alloys at high
temperatures under dwell-loading conditions. Evaluation of the effects of service-representative
loadings on the failure mechanisms and lifetimes observed is a continuing need for materials
and component lifing research programmes. The high cost of manufacture and the fracture
critical nature of operation emphasises the need for long lifetimes that are determined using safe
and conservative life-prediction methods without the unnecessary and uneconomic use of
expensive materials.

The objectives for the design engineer are to meet the needs for improved rotor stability and
improved performance, whilst reducing the first part and life-cycle costs associated with turbine
disc manufacture and overhaul. Cost reduction during manufacture is a prime motivation for
continued development of component assembly initiatives, such as fully welded rotor drums and
the potential introduction of advanced high strength steels.

In order to meet the highest operating temperatures and the component stress levels demanded,
it has been necessary to develop a series of progressively higher strength steel and Ni-based
superalloys, such as IN718, Waspaloy and U720Li. These are generally manufactured using cast
and wrought processing. However, the complex chemistry of these alloys makes production of
segregation-free ingots very difficult. A triple-melt process is necessary, involving vacuum
induction melting (VIM), electroslag refining and vacuum-arc remelting (VAR) to limit
macrosegregation and defect inclusion. Manufacture of larger components, or more complex
alloys, would necessitate a change to atomised powder processing to limit segregation, whilst
dual alloy processing offers the potential for overcoming the variability in strain distribution
across the section of large forged turbine wheels.

To meet the demands for improved technical capability and higher operating efficiencies for
small to medium engines, dual alloy and, potentially, integrally bladed disc (blisc) technologies
are being developed. A dual alloy disc enables the differing mechanical property requirements
of the hub and rim regions to be reconciled within a single disc structure by combining suitable
materials that meet the differing strength-temperature property requirements. This offers
considerable advantages over the conventional counterpart in terms of higher temperature and
component size capabilities, allowing substantial power and efficiency gains. Recent advances
in Europe and in the USA have demonstrated the practicability of joining dissimilar materials to
produce small aeroengine discs4,5; however, existing knowledge on the success of these joining
routes in producing large-scale components and high quality joints is limited by the
manufacturing technology. This is currently being developed in conjunction with validated
qualification and NDT procedures and lifing methods. Further development and implementation
of advanced manufacturing methods will continue to be a high priority for turbine disc
applications.

One of the major concerns with the introduction of powder processed materials into service is
the potential presence of a range of defects and inclusions in the final component. These
originate from a number of sources including furnace linings and ceramic spray nozzles, and
generally fall into two categories: i) hard alumina type, ranging in size from 100-200 µm and
300-400 µm; and ii) soft silicate type, ranging in size from 50-100 µm and 200-300 µm. These
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defects have been shown to have a deleterious effect on crack initiation lives and sieving of the
powder does not guarantee their removal. To allow powder alloys to be fully utilised a
probablistic lifing methodology is required to describe the crack growth behaviour from defects
under service conditions and enable a safe, economical service life to be declared. If a high
strength powder alloy is to be used to its full capability, then the majority of the life limiting
defects in the material will be smaller than the current NDI capability and are, therefore,
effectively undetectable. These defects can act as crack initiation sites at the stress levels to
which the components will need to operate to justify their selection and, therefore, they may
control the service life of the component. To date, lifing methods assume that the defect acts as
an equivalent sized crack, which forms on the first loading application and hence, they lead to
overly pessimistic service lives. Previous work indicates that the first-cycle crack formation
method is often incorrect. An initiation life is associated with many of the defects present,
which is sufficient to allow economic use of powder materials for industrial gas turbine
applications.

Turbine Seals

Containment of the hot gases delivered from the combustor is crucial to the efficiency and
optimum running of the gas turbine. Both honeycomb and brush-sealing systems have been
designed to limit losses and maximise the energy captured by each stage and prevent damage
being caused to other components. For example, sheet materials such as H214 and PM2000 are
used to manufacture fine-scale honeycomb sealing strips to prevent hot-gas leakage over the tips
of the rotor blades. Improvements to this technology have been targeted with the application of
protective and abradeable coatings to improve high temperature oxidation, fatigue and wear
resistance. These structures suffer severe temperature loadings such that the oxidation behaviour
during extensive engine running and the wear properties of the material in contact with the rotor
seal fins is crucial to their successful application.

Turbine Coatings

Surface engineering and coatings technology plays a crucial role in the operation of all high
temperature plant, particularly for GT engines. The desire for higher operating temperatures,
improved performance, extended component lives and cleaner more fuel-efficient power
generation places severe demands on the structural materials used and many components
operating at high temperature are coated or surface treated to enable cost-effective component
lifetimes to be achieved. A detailed review of surface engineering technology has been provided
elsewhere6.

Coatings are generally applied to provide oxidation, corrosion or thermal protection depending
on the nature of the operating environment and thermal loads to be endured. Any coating should
possess the requisite mechanical properties, adhesion and metallurgical stability in contact with
the substrate to withstand the thermomechanical cyclic loadings imposed. The main types of
protective coatings used for gas turbine components can be defined as follows:

•   Diffusion Coatings: Formed by the surface enrichment of an alloy with either aluminium
    (aluminised), chromium (chromised) or silicon (siliconised). In some systems combinations
    of these elements are possible i.e. chrome-aluminised or silicon-aluminised.

•   Overlay Coatings: These are commonly known as MCrAlY coatings (where M is the base
    metal, normally Ni or Co or a combination of the two; Cr is chromium, Al is aluminium and
    Y is yttrium) and represent a family of corrosion resistant alloys designed for high
    temperature surface protection.

•   Thermal Barrier Coatings (TBC’s): These coatings are designed to insulate the component
    from the hot gas path and generally consist of a ceramic top-coat attached to the metal

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    substrate by means of an oxidation resistant bondcoat (typically an MCrAlY or a diffusion
    aluminide coating).

Diffusion coatings are applied using a range of techniques, including pack cementation, slurry
cementation, pack and gas-phase chemical vapour deposition (CVD) and metallising. The
processes that are most widely used are 'aluminising' and 'chromising'. Recent developments
have focused on over-pack CVD and gas-phase CVD to produce low activity coatings (more
ductile and higher purity) that enable internal features such as cooling passages to be coated.
Typically, aluminide coatings are deposited to thicknesses of between 30-100 µm depending on
the type of aluminide formed. Under severe hot corrosion conditions, or at temperatures above
1100°C, aluminides offer limited protection and, therefore, modified aluminide coatings
offering improved oxidation/corrosion resistance have been developed. These include additions
of Cr, Si, Ta, Hf, Zr, Y and precious metals, such as Pt and Pd, made using co-deposition,
electroplating or surface pre-treatment of the substrate. The most significant advance in this area
has been the development of platinum-modified aluminides (LDC-2, RT22LT, CN91). Figure
3.4.12 shows the microstructure of the RT22LT variant. Technology development programmes
continue to target further improvements to aluminide systems with additions of Pt-group metals
and improved deposition methods to provide improved protection and durability levels for
complex shaped parts.

Overlay coatings are based on an M-CrAl-X system, where M is Ni, Co or a combination of Ni
and Co, and X is an oxygen active element, for example Y, Si, Ta or Hf. The composition is
selected to provide a balance between oxidation and corrosion resistance and coating ductility.
The active element addition(s) enhance oxide scale adhesion and decrease the oxidation rate.
NiCrAlY coatings are generally the most oxidation resistant, whilst CoCrAlY systems provide
good hot corrosion resistance. These coatings are most often applied using thermal spraying
techniques such as plasma spray or high-velocity oxy-fuel (HVOF) processes, typically to a
thickness of between 125 and 300 µm. Also, electron beam-physical vapour deposition (EB-
PVD), electroplating and laser fusion methods have been used. Plasma spraying has the
advantage of high deposition rates generating homogeneous coatings with microstructures
consisting of fine equiaxed grains. However, the process is “line of sight”, requiring complex
robotic manipulation for complete component coverage. Figure 3.4.13 shows an example of an
argon shrouded plasma sprayed LCO22 (CoNiCrAlY) overlay coating.

There is considerable interest in the further development and application of HVOF to deposit
MCrAlY coatings due to the high densities (< 1% porosity) and adherence of the coatings
produced with less degradation of the powder during spraying. Continued development of
overlay protection systems has targeted improved process control and advanced multi-layer
“smart” coating compositions that are able to adapt to the temperature and corrosive
environment endured. Other “smart” coating systems have also been proposed that provide self-
diagnostic capabilities to indicate surface temperature and stress levels using laser fluorescence
spectroscopy or piezo-spectroscopic methods. Further work in this field is needed to develop the
techniques and establish methods for wider application in the field.

A TBC typically consists of a metallic MCrAlY or aluminide bondcoat layer applied to a nickel-
based substrate and a yttria partially stabilised zirconia (YPSZ) ceramic thermal insulation top-
coat layer, as shown in Figure 3.4.14. The bondcoat’s function is to provide ceramic adhesion
and oxidation protection to the substrate, as the ceramic is essentially oxygen permeable.
Selection of a 6-8wt.% YPSZ ceramic is based on the high temperature tetragonal phase
stability coupled with low thermal conductivity and a high melting point, and its inertness with
respect to the bondcoat and substrate alloys. A relatively high coefficient of thermal expansion
(approximately 70% of the substrate) enables accommodation of thermal compressive stresses
developed on cooling and makes YPSZ an ideal candidate for TBC systems.



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Air plasma spray (APS) or electron beam physical vapour deposition (EB-PVD) methods are
used to deposit the ceramic top-coat depending on the component type and duty cycle. The
properties of the TBC are largely governed by the processing method used to apply the ceramic
and a number of key properties such as thermal conductivity, weight gain, erosion resistance,
mechanical behaviour (strain tolerance) and aerodynamic efficiency (surface roughness) must
be considered and optimised for a particular application. Of equal importance to the durability
of the coating system is the interaction of the bondcoat with the environment and the substrate
alloying additions which combine to influence the thermal and time-dependent stability of the
interface. Significant effort is still needed in characterising the influence of the IGT loading
conditions on the long-term stability and mechanical and thermal properties of TBC systems.

Technology development for TBCs has focussed primarily on improving the thermal properties,
phase stability and resistance to sintering of the ceramic top-coat at temperatures above 1250°C.
The development of more accurate lifetime prediction models for multi-layer coating systems
has been combined with a significant effort aimed at evaluating the mechanical and thermal
behaviour of coated superalloy specimens under creep, fatigue and thermomechanical fatigue
loading conditions, as well as service-representative corrosion and oxidation testing. The
development of higher performing thermal barriers (reduced thermal conductivity and increased
integrity), materials characterisation (including mechanical properties), remnant life assessment
(including NDE) and lifing methods will be a continuing need for the IGT designer.

Summary of Component Materials Challenges

From the preceding sections, the following items have been identified as the key targets and
materials- enabling technologies for the wider implementation of cleaner coal combustion
systems for gas turbine applications:

•   Advanced manufacturing and joining (cost-reduction, increased performance and integrity).

• IGT specific alloy development (corrosion resistance, stability, properties, casting/yields,
costs).

•   Advanced materials and composites (CMCs, ODS alloys, TiAl alloys).

• High strength nickel and steel technology for rotor discs (melting, forging, welding, NDT,
lifing).

•   Develop data generation and lifing methods (deformation models and damage algorithms).
       _ Combustors
       _ Blading
       _ Discs
       _ Coatings

•   Design by modelling (processing through to mechanical properties and lifing)

•   Overhaul and repair, remnant life assessment.

• Improve properties and processing methods for diffusion, overlays and thermal barrier
coatings.

•   Improved multi-layer “smart” coatings and thermal barriers (thicker, strain-tolerant, thermal
    conductivity, stability, sintering).



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Materials Sources/Component Suppliers

Component suppliers for gas turbine engines are based throughout the world. Many of the
companies that supply components such as blades, vanes, discs and casings to the engine
manufacturers are owned by multi-national organisations that supply to both the land-based and
aeroengine sectors of the market. The majority of these companies have manufacturing facilities
in the UK and Europe, though a significant number are based within the USA, which impacts
upon manufacturing costs and availability as well as the expertise and capabilities base within
the EU. Increasingly there are issues associated with technology transfer to European engine
manufacturers for materials developments funded by DOE and DOD programmes in the USA.
The following represents a brief list of typical component suppliers:

Howmet Ltd:                      turbine blade and vanes castings
AETC:                            turbine blade and vanes castings
Wyman Gordon:                    disc forgings, shafts
Firth Rixson:                    disc forgings
Haynes International:            nickel alloys
Special Metals:                  high temperature alloys
Canon Muskegon:                  high temperature alloys
Ross & Catherall:                high temperature alloys
Praxair:                         coatings supplier
Sultzer Metco:                   coatings supplier
Sermatech:                       coatings supplier
Chromalloy:                      coatings supplier


3.4.3   Review of UK Opportunities

UK Strengths and Weaknesses

The UK power generation industry benefits significantly from established high levels of
expertise in the fields of design, engineering and materials technologies. Experience has been
developed over many years in the design and manufacture of sophisticated gas turbine plant and
new innovative operational cycles to meet the demands of operators, users and legislators
providing a valuable resource for the UK economy. These skills should continue to pay
dividends by providing flexible, commercial systems and engineering expertise to meet the
needs of a range of customers in the global power generation business.

This know-how has been developed, however, within a number of organisations that, put quite
simply, no longer exist due to changes in the structure of the power generation business, i.e., the
equipment manufacturers and operators, the supply industry and the governmental support and
infrastructure for the power network and the associated technology development centres. Some
believe these changes have to some extent resulted in an over reliance on the
University/academic system in providing technology R&D, but also makes it incumbent on the
power sector businesses to ensure that the requisite skills and expertise levels are maintained
and developed further. Continued success for industry is dependent on the development of
human resource skills at the University/college level and the subsequent recruitment of high
quality engineering and materials graduates and technicians into the gas turbine manufacturing
and power generation businesses. Failure to address the development of the educational and
industrial experience for students and personnel will limit the ability of the industry to remain
flexible and at the forefront of the technology.

The development of materials technologies for the IGT business has been over dependent on the
pull-through of technologies from the aero-engine sector. The aero-engine sector has provided
much useful technology over a number of years but it has been recognised by many within the
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industry that this dependency has failed to resolve a number of specific IGT issues that still need
to be addressed and have been discussed in previous sections. Whilst much of the technology is
transferable it is necessary for the IGT technology development sector to maintain a degree of
independence and lead programmes that target the specific needs of the land-based systems.

Manufacturing and materials supply within the UK for the IGT business is declining. Many of
the suppliers remaining within the UK are now owned by multi-national or foreign companies
and often act solely as manufacturing facilities with a limited remit within their organisations
for technology acquisition and research and development. This impacts upon the ability to
identify cost and risk-sharing partners for innovative research, both within Universities and
industry. Finding common agreement for collaboration and ownership of intellectual property
rights (IPR) is increasingly more difficult and time-consuming. That said, the suppliers within
the UK industry have established a world-renowned expertise for flexibility and capability to
deliver components to quality and cost.

The UK power generation industry continues to be affected by the changes to the way in which
government supports the power infrastructure. For example, closure of the CEGB and
associated research centres and the privatisation of the coal and power generation and supply
businesses have limited the number and scope of technology development programmes. As a
consequence, the level of government support and funding for technology and materials
development in the UK has diminished significantly and needs to be addressed if the UK
industry is to remain competitive with manufacturers in the US and the far-east.

Review of UK R&D Capabilities

Without doubt the UK possesses a World-class level of expertise in both Industry and
Academia. A fact recognised by students, researchers and engineers who come to the UK to
study and work and benefit their own expertise levels. There is an established University
research network with a number of departments benefiting from, for example, IRC and Faraday
infrastructure funding, regional grants and support from industry in the form of Centres of
Excellence (CoEs) and University Technology Centres (RR-UTCs). University departments
benefit significantly from a variety of funding sources such as EPSRC, which is geared to some
extent with support from Industry (both actual and in-kind). Other sources of income include
MOD Joint Grant Scheme. There is no equivalent to the RR-UTC system within the IGT sector,
though certain University departments, such as Cranfield, have established IGT relevant CoEs.
Likewise there is no equivalent of the MOD JGS for supporting/prioritising IGT-relevant
research.

Beyond the University/academic system the R&D capabilities for the UK have declined
significantly. There is increasingly limited support for government-owned research centres apart
from those seen as core to the requirements of the MOD. Organisations such as CEGB, NPL,
NEL and DERA (now QinetiQ) have either been closed down or have changed significantly
over recent years with more limited access to government funding etc. The technology centres
associated with the now privatised utilities have also undergone some degree of rationalisation,
if they have managed to survive at all. Technology development has been left largely to Industry
with contractual and consultancy support from test and design companies, some formed from
the previously government owned facilties, eg., QinetiQ.

Increasingly it is incumbent upon the manufacturers to identify, co-ordinate and support
technology development programmes and make best use of the facilities (University and
otherwise), partnerships and funding (EPSRC, DTI, COST, EU Framework Programmes)
available within the UK and EU. The global market for power generation demands further
development of coal-fired combined cycle plant whilst satisfying the environmental legislation
needed to mitigate the risks associated with climate change and pollution. Government support
for these technologies is vital if UK industry is to maintain and develop its market share.
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Market Opportunities

The US, EU and Global power generation markets for coal and gas-fired IGT systems continues
to offer strong opportunities for the UK manufacturing sector. Also, there are increasing
demands for the development and installation of renewable fuel-fired systems based on biomass
technology that are often linked to the establishment of net-zero CO2 emission cycles. Natural
gas-fired technology will continue to offer significant opportunities for UK manufacturing with
increased demand for dual fuel capability (gas and oil). However, the most significant sector for
the market, globally, is expected to be coal-fired systems. This represents the most widely
available fuel source for many non-industrial/developing nations. The development and support
of cleaner coal technology programmes is therefore key to the success of the UK power
generation industry in capturing a significant section of this growing market.

Further opportunities for UK industry continue to arise from the high level of expertise
available. This is demonstrated in the export of design and engineering technologies within the
land-based and aero-engine sectors of the market that, increasingly, include risk and revenue-
sharing partnerships, as well as project management and maintenance of large-scale
installations. It has been recognised world-wide that the capabilities within the UK for design,
manufacturing and systems engineering (controls, software, component design, project
management, etc) are at the forefront of these technologies.

The component supply industry within the UK continues to offer opportunities for employment
and revenue- generation, providing significant benefits to the UK economy. Despite the
continued decline in the number of people employed and an increase in the numbers of
companies owned by multi-national or foreign organisations, there are a significant number of
companies involved in the supply of forgings, castings and coatings to the engine
manufacturers. These trends form part of a wider shift within manufacturing and are seen within
other sectors, such as automotive, and arise from the need for companies to consolidate and
streamline their businesses in order to remain competitive and survive.


3.4.4   Future R&D Priorities

From the foregoing sections it is clear that there is a continuing and growing need for political
and financial support for technology development programmes in the field of high temperature
materials and component systems for industrial gas turbine power generation systems.

Development of advanced manufacturing and joining processes in conjunction with computer-
based modelling is crucial to the needs of the industry to enable cost-savings during
manufacture to be gained in conjunction with increased performance and integrity levels. The
increased use of advanced joining technologies, such as electron-beam and friction welding, as
well as near net shape, novel fabrication methods such as laser-deposition and powder
processing of high strength materials are seen as vital to meeting these objectives. Development
of computational based process modelling methods is needed to limit the costs associated with
the advent of new design and processes and provide increased flexibility. The development of
through-life computer based systems is seen as vital to future capabilities. In the limit, these
would establish a series of linked computer based models capable of simulating the processing
methods (e.g., solidification, forging and machining), the subsequent properties, as well as
defect and residual stress distributions, and the effects of the duty cycle on lifetime usage for
particular components.

Current GT power generation systems are performance-capability limited by the alloys used for
the hot gas path components for these engines. For future engine technology this will continue
to be an increasing need and will only be addressed with the development of IGT specific alloys
                                           90 of 108
(conventional, DS and SC) capable of providing the requisite mechanical property, corrosion
resistance and stability levels using cost-effective manufacturing. Further development of high
strength alloys (steel and nickel based) and dual alloy technologies for compressor and turbine
rotors will continue to be a priority. The development of computational tools to predict the
effects of alloying additions on the phase stability, processing and properties is crucial to these
objectives and to limit the excessive costs currently associated with alloy development
programmes. Failure to address these issues will severely limit the future capabilities in terms of
the operational cycles and efficiencies that can be gained from gas turbine technology. Low
density materials (intermetallics and composites) still possess the potential to provide step-
change materials solutions to reducing rotor mass, whilst enabling harder working, more
efficient stages within both the compressor and turbine to be designed. Also, the thermo-
physical and mechanical property characteristics of ceramic composite materials have yet to be
exploited fully in providing solutions for components such as combustors. This technology is at
a more advanced stage within the USA.

Developing an improved understanding of the mechanical properties and behaviour of
substrates and coatings under service-representative loading conditions and environments
continues to be a key requirement. Combined with these activities is the development of more
advanced life assessment methods for hot section components operating under creep-fatigue-
corrosive loading environments and improved methods for data analysis and database systems.
Research within both industry and academia would provide benefits by enabling more accurate
lifetime prediction and, thereby, fuller utilisation of expensive materials and components and is
the key to the more effective operation of power generation plant. Optimised design and lifing
procedures will enable reductions in first-part manufacture and operational costs and provide
benefits across the industry to manufacturers and operators alike.

Development of advanced Non Destructive Inspection (NDI) techniques and the further
development of remnant life assessment and repair/overhaul methods are also required to
provide through-life cost reductions from more optimum and safer component usage. This is
particularly true for high strength fracture critical rotating parts, such as discs and shafts, and
hot section components and protective coatings exposed to high temperatures for long periods.
Methods for in-service inspection of, for example, thermal barriers such as optical spectroscopy
that can be linked to the health and usage monitoring systems are seen as key to providing
significant benefits in life-usage management.

New, improved coating systems are required that use cheaper more efficient, non-line-of-sight
deposition methods, providing high integrity protection from oxidation and corrosion. Coatings
capable of withstanding the aggressive environment within coal-gasification plant, as well as
“smart” multi-layer coatings that modify their phase development to meet the local
environment-temperature loadings are needed. In addition, future coatings will be required to
emit, for example, fluorescent or spectroscopic signals for life management detection systems.
The specification, behaviour and processing of such systems remains to be achieved though
preliminary work in the UK and USA has demonstrated the feasibility of these concepts.
Improved thermal barriers are needed that are capable providing increased thermal protection
and more reliable integrity for longer periods than are currently achievable (thicker layers,
strain-tolerance, reduced thermal conductivity, increased phase stability and resistance to
sintering).

References for Section 3.4

1. D.H. Allen, S. Beech, L. Buchanan, J. Oakey and R. Vanstone, “Requirements for Materials
   Research & Development for Coal-Fired Power Plant:- Into the 21st Century.” September
   1997.
2. IEA World Energy Outlook 2000.


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3. G.J. Kelsall and J.K. Whinfrey, “Developments in Low Calorific Gas Fuel Combustion.”
   Proc. 6th Int. Conf. On Technologies and Combustion for a Cleaner Environment, Porto,
   Portugal, July 2001.
4. Y. Bienvenu et al., “Diffusion Bonding of Nickel Superalloys to Manufacture Turbine
   Components with a Graded Microstructure.” FGM 1994, EPFL Lausanne, 1995, p. 487.
5. Y. Bienvenu et al., “Assemblage par Soudage Diffusion de Deux Superalliages.” J. de
   Physique III, 4, 1994, p. 117.
6. J.R. Nicholls, “Design of Oxidation-Resistant Coatings.” Journal of Materials, January
   2000, pp.28 – 35.




                   Figure 3.4.1. ALSTOM Power Industrial Gas Turbine




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Figure 3.4.2. Typical example of a conventional multi-stage compressor for an intermediate
sized engine (ALSTOM Power (UK) Ltd., Typhoon).




Figure 3.4.3. Large scale IGT compressor and turbine (ALSTOM Power (CH) Ltd., GT engine).




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Figure 3.4.4. ALSTOM Power G30 family of combustors.




          Figure 3.4.5. CMC combustor liner




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