Mat UK Energy Materials Review R&D Priorities for Steam Turbine Based Power

Transcription

1 Mat UK Energy Materials Review R&D Priorities for Steam Turbine Based Power Generation. 3rd August 2007 Prepared by: S Osgerby - ALSTOM Power Send comments to: steve.osgerby@power.alstom.com Page 1 of 11

2 1. EXECUTIVE SUMMARY INTRODUCTION STEAM TURBINE MATERIALS High Pressure (HP) and Intermediate Pressure (IP) Cylinders Rotor Forgings Casings and Valve Chests Blading Sealing Valve Internals Bolting Low Pressure (LP) Cylinder Rotor Forgings Casings Blading R&D PRIORITIES Current R&D Programmes Europe UK Japan USA Future R&D Priorities High Temperature Low temperature Underpinning Activities...11 Page 2 of 11

3 1. EXECUTIVE SUMMARY. This report summarises the current status of material use in steam turbines. Future materials requirements are identified and current collaborative and public-funded materials research is reviewed. The following key recommendations are made for future research programmes into materials for steam turbine applications: Development of alloys with improved creep resistance at higher temperatures; Development of alloys or surface engineering technologies with steam oxidation, erosion and wear resistance at higher temperatures. Development and long-term characterisation of improved welding consumable for high temperature service Development of alloys with improved combination of strength, fracture toughness and stress corrosion cracking resistance Improved understanding and predictive modelling of degradation and damage mechanisms at both high and low temperature Non-invasive inspection techniques for in situ assessment of material condition Test procedures for miniaturised specimens that can be directly correlated with tests results from standard sized specimens Facilities for extended duration exposures to environments that reproduce service conditions eg high pressure steam, solid particle erosion etc Page 3 of 11

4 2. INTRODUCTION. Steam turbines are an essential part of combustion-based power generation technologies. At some point in the cycle, the majority of these technologies raise steam whose energy is converted via a steam turbine. The materials issues related to the steam turbine depend largely on the temperature and pressure of the steam generated and are almost independent of the steam raising technology employed. At the current time steam turbines serve two principal fossil-fired technology markets: Coal or lignite-fired plant based on PF or CFB combustion technology. For utility-scale generation this may require steam turbines with outputs up to 1100MWe. Gas-fired plant incorporating a combined cycle where the exhaust gas from a gas turbine is used to raise steam. To match the lower power output of the gas turbine, steam turbines for these applications have outputs typically in the range up to 350MWe. In the 1990s the second of these markets was dominant. The dash for gas in the UK was replicated to a greater or lesser extent in markets around the world, especially in the North American market. More recently concerns over security of supply and over the volatility of gas prices have led many utilities, both in Europe and the USA, to reexamine the potential for coal-fired generation. In the UK both E.ON and RWE have announced plans for new coal-fired plant, operating at state-of-the-art (>600 o C) steam conditions, at Kingsnorth and Tilbury respectively. This follows orders placed for similar plant by their parent companies in Germany. Orders for such plant have also been placed by US utilities. The enormous growth of generating capacity in China has also been largely based on the construction of coal-fired plant. In addition to the construction of new plant, an important subset of the first market is retrofit turbines. Advances in the internal efficiency of steam turbines have made it economically attractive to replace elements of older turbines with improved modern turbine components. The improvements can either reduce fuel consumption or increase power output. Steam turbine development trends fall into two categories: 1. Engineering led: Improvements in the internal efficiency of the steam turbine. These developments largely focus on improved aerodynamic performance, sealing and cylinder configuration. 2. Materials led: Improvements in the cycle efficiency. These developments focus on increasing steam temperatures and pressures so as to increase the thermal efficiency of the overall cycle. Engineering-led improvements bring benefits to the whole range of technologies served by the steam turbine and they have provided the basis for the current retrofit market. Advanced materials have also played, and continue to play, a role in this area, for example through: The introduction of stronger or less dense blading alloys enabling more efficient steam path arrangements. Page 4 of 11

5 The introduction of rotor materials with the necessary combination of high temperature creep resistance and low temperature fracture toughness to enable the construction of single-flow, single cylinder steam turbines. Materials-led improvements can be sub-divided into high and low temperature applications. At high temperature they are focused much more on steam turbines for coal-fired plant where there are clear advantages in elevating temperature and pressure to increase thermal efficiency and reduce emissions. In combined cycle plant, the drivers for increased temperature in the steam turbine are low. Here the steam turbine inlet temperature is largely dependent on gas turbine exhaust temperatures and there is no great thermodynamic benefit in elevating this exhaust temperature. Currently these steam turbines operate with inlet temperatures of up to 585 C but there is no great driver for increasing this temperature. Improvements in temperature and pressure capability for coal-fired plant are almost entirely dependent on improvements in materials technology through: Development of alloys with improved creep resistance at higher temperatures; Development of alloys or surface treatment technologies with steam oxidation resistance at higher temperatures. The drivers for steam turbine development for current coal gasification based power generation cycles are similar to those for gas-fired combined cycle plant, where superheating is limited to the exhaust gas from the gas turbine. However, if methods of superheating in the high temperature fuel gas or supplementary superheating stages are provided, then these systems could take advantage of the higher steam conditions being used in PF plant. Materials development for low temperature applications applies to all plant types and irrespective of component concentrates on producing the required combination of strength, fracture toughness and stress corrosion resistance at specific localities. 3. STEAM TURBINE MATERIALS. The following sections review the materials used in the different parts of modern steam turbines. Within each chamber the function of individual components such as rotors, blades etc., is identical, although the operating conditions and hence material requirements varies significantly. 3.1 High Pressure (HP) and Intermediate Pressure (IP) Cylinders Material requirements in the HP and IP cylinders depend critically on the steam inlet and reheat temperatures respectively. For the current generation of steam plant being constructed in Japan and Europe these temperatures are up to 620 o C. Materials research is required for plant that will operate at steam inlet temperatures up to 760 o C Rotor Forgings Current rotor forgings are based on 9-10% CrMoVNbN steels, the main alloys currently being applied having either a Mo addition of 1.5% or an addition of up to 1.0%W in partial substitution of the Mo content. V and N contents have been Page 5 of 11

6 optimised to provide precipitation strengthening through a dispersion of VN particles and a low level of Nb is incorporated to control grain size during high temperature heat treatments. For the very highest temperature applications, additions of boron are being made Casings and Valve Chests Castings for valve chests and cylinder casings exploit analogous alloys, generally with lower C content to provide improved weldability Blading Blading alloys for operation up to ~600 o C are similar to the rotor forging alloys. However at higher temperatures oxidation becomes an issue: martensitic steels containing higher chromium contents do not possess sufficient creep strength so austenitic alloys are used. As steam inlet temperatures rise to 650 o C and above these alloys will require coating to achieve the necessary oxidation resistance. For plant operating at >700 o C Ni-base alloys will be required to achieve both the strength and oxidation requirements. Solid particle erosion can be an issue and erosion-resistant coatings are required to alleviate this problem Sealing Current ring and brush seals of steam turbine power generation plant are limited to operation at temperatures below C by the capabilities of the materials used. Above this temperature range excessive distortion and wear results in efficiency losses and poor performance that impact upon component design, declared lifetimes and costs of manufacture and operation. The materials requirements necessary to establish the next generation sealing systems capable of operating at 650 C and beyond are: higher temperature creep strength to prevent loss of sealing due to distortion and enable longer lifetimes for components operating under extreme temperatures and pressures high temperature resistance to steam oxidation and wear (use of hard facing treatments) providing lubricant-free abrasion resistance and high load bearing capability effective use of materials in demanding environments providing reduced costs due to improved design, manufacturing and longer periods between overhaul and applicable to retrofit / upgrade of power generation plant Valve Internals The key issues for valves are sliding wear and solid particle erosion. Abrasionresistant coatings or welded inserts are used on a regular basis in current plant Bolting Meeting the requirements of bolts operating at the very highest temperatures has frequently required the exploitation of Ni-based alloys such as Nimonic 80A or Refractalloy 26. Even higher strength alloys will be required for >700 o C. Page 6 of 11

7 3.2 Low Pressure (LP) Cylinder Material requirements for the LP cylinder are primarily dictated by the need to avoid cracking due to stress corrosion cracking (SCC) and fatigue. Steam entry and exit temperatures are typically 250 o C and 40 o C respectively Rotor Forgings LP rotors are usually manufactured from low alloy NiCrMoV steel. Designs may be either monobloc or welded construction. A key requirement is to avoid SCC in blade attachment areas. Approaches used to achieve this are either control of material strength or local surface treatment to introduce compressive residual stresses in critical areas Casings Casings are not generally highly stressed. Carbon steel or cast iron is usually used subject to flow accelerated corrosion resistance being acceptable Blading Blading requirements vary along the steam flow of the LP turbine. Blades near the inlet are relatively small and operate in dry steam. Near the outlet the blades are much longer and operate in steam that contains significant moisture. Precipitation hardened stainless steels are usually used for the longest blades although titanium alloys have acceptable properties (albeit much higher cost). Last stage blades may be subject to water droplet erosion. The leading edge of the blades may be modified for increased erosion resistance through cladding, welded inserts or local hardening. 4. R&D PRIORITIES Current R&D Programmes Europe The UK is active in a number of major EU projects for development of steam turbine materials. The COST 536 programme covers a wide range of power generation technologies but one of its major themes is Steam Power Plant (SPP) within which there is a major activity on steam turbine materials. It includes most European boiler and steam turbine makers and their materials suppliers. The programme started in 2004 and runs to As for the boiler area, UK participants in the steam turbine area are also supported via the DTI s Technology Programme. The COST 536 SPP activity s main objective is the development of materials technology to enable operation of power plants with steam temperatures of 650 C. A parallel programme, COST 538, focuses on Plant Life Extension. The programme includes materials and coatings for steam turbines, gas turbines and boilers and is improving understanding of degradation mechanisms in high temperatue ferrous alloys. Page 7 of 11

8 In the EU AD700 project, launched in 1998, the first three years resulted in confirmation of the technical and economic feasibility of the concept, which fundamentally depends on the application of nickel-based alloys in steam turbine parts to enable steam temperatures of C. The application of Ni-based superalloys at steam temperatures up to 700 C/375 bar will give an overall thermal efficiency of up to 55%, compared with the 47% efficiency of state of the art 600 C/300 bar/300 bar double reheat plant. This is expected to reduce fuel consumption and thus CO 2 emissions by around 15%. Much larger components are required for steam cycle plant than for gas turbines so these temperature increases represent a significant materials challenge. Turbine-related activities included the assessment of a range of candidate alloys and the manufacture of the first prototype components. The second phase, which was completed at the end of 2006, included the extension of the prototype demonstration and characterisation programme. The success of the AD700 project encouraged the launch of a demonstration programme involving the operation of critical boiler and turbine valve components in an operating power station in Germany. This COMTES 700 project is supported by a consortium of European utilities and by the EC. The demonstration components have been successfully operating at 700 o C since July Nationally funded steam turbine development programmes in Europe, outside the UK, are also of importance. In Germany in particular, there is strong national funding for the COORETEC and MARCKO programmes, and the VGB is funding the long term characterisation of advanced 9-12%Cr steel components UK The UK DTI Technology Programme provides support for several steam turbine activities as well as parts of COST 536. Current projects include: Alloy Development For Critical Components On Future Coal-Fired Power Plant High Temperature Sealing for Advanced Super Critical Steam Turbine Plant Advanced Materials For Low Pressure (LP) Steam Turbines Improved Modelling of Materials for Higher Efficiency Power Plant The DTI also funds UK participants in a project allowing collaboration between UK and US organisations. Activities include steam oxidation and microstructural degradation of alloys used in steam turbines. The DTI National Measurement System Materials Programme has a rolling programme that includes projects, carried out at NPL, which support steam turbine development: One current project, Key measurements on in-situ oxide scales to ensure future energy security includes research into oxidation mechanisms on a range of boiler and steam turbine alloys. This project began in April 2007 and is planned to run for three years. There is potential for new projects relevant to steam turbines to be launched in future years. The EPSRC Supergen II Programme on Plant Life Extension funds work, supported by an industrial consortium, at four UK Universities. This programme includes tasks on Condition Monitoring, Microstructural Degradation and Modelling of Mechanical Behaviour. Page 8 of 11

9 4.1.3 Japan There are also significant development programmes outside Europe. In Japan NIMS is responsible for a major programme focused on the continued development of 9-12%Cr steels for 650 C steam conditions, the same principal objective of COST 536. A feasibility programme funded by the EPDC whose objective was 700 C steam conditions was carried out in and in 2006 METI and NEDO launched a materials development project with objectives of up to 800C USA In the USA the DOE is funding research on steam turbine materials for temperatures up to 760C. The programme mainly addresses the development of Ni-base alloys and coatings. 4.2 Future R&D Priorities High Temperature As a result of the trends in steam turbine development, the major emphasis of materials development around the world is focused on improved high temperature materials for the high temperature cylinders. High temperature materials development is a very long term procedure. A typical development programme proceeds through several stages: Investigation of trial melts including creep testing to at least 10,000 hours (~2 years); Manufacture of a prototype component in the best trial melt (~1 year). This stage is essential to demonstrate that the alloy can be applied to the very large components required for turbine rotors and casings without problems arising from excessive segregation or cracking during manufacture. The inspectability of large components is also demonstrated. Characterisation of the prototype including creep testing to at least 30,000 hours (~4 years). There is potential for significant variation of properties through the section of large components, especially in comparison to the properties achieved in trial melts. Therefore characterisation of the prototype is essential. Characterisation typically includes long term creep testing, low cycle fatigue and cyclic hold testing and investigation of the influence of long-term ageing on tensile and impact strength. Investigations of fracture toughness and creep crack initiation and growth properties may also be carried out. Launch as commercial material and gain first purchase order (~1 year); Test commercial products to establish scatterband of properties (~4 years). Cast to cast variation in properties is typically of the order of +/-20%. A knowledge of this scatterband and the position of the first prototype within it is required to fully exploit the properties demonstrated in a single prototype. It thus takes about 12 years to achieve a completely reliable, mature material. For example in Europe the development of a new generation of martensitic stainless steel materials for high temperature rotors began in the COST 501 programme in After investigation of trial melts, two prototype rotors were manufactured in Long term characterisation of these materials led to their selection for the Skaerbaek and Nordjylland power plants in Denmark in These machines were Page 9 of 11

10 manufactured and commissioned in Test pieces from the rotor forgings used for these turbines were included in a long-term testing programme funded by VGB which was launched in This programme finished in 2001 with the issue of data establishing a scatterband of properties for commercial rotors manufactured in these alloys. The total time elapsed from launch of the development programme to this fully mature condition has been 15 years. Attempts are being made to reduce this development time. Metallographic studies have been carried out and models are being developed to predict microstructure and the long-term properties that depend on them. One key area of success in this effort has been the development of models predicting thermodynamic equilibria: MTDATA developed by NPL in the UK and Thermocalc developed in Sweden. An alternative approach is the use of neural network modelling to assess the influence of changes in chemical composition and heat treatment. These tools offer the ability to reduce the number of trial melts to be assessed before finding an alloy meeting the objectives and they also offer support for longer term extrapolation of data, increasing confidence so that alloys may be exploited earlier in the development cycle. Nonetheless the models are not yet sufficiently accurate or robust to remove the need for long-term testing. Even where long term data already exist on alloys, it is possible that new applications require a cycle of prototype demonstration and characterisation. For example the application of nickel-based alloys to steam turbine design and manufacture is supported to some extent by the existence of long-term creep data on some of the candidate alloys. However, these data were derived from product forms such as sheet and small diameter bars whereas turbine manufacture requires components with sections in the range mm and weighing many tons. The difference in size has a potentially very significant influence on the microstructures and thus properties that are achieved. The greater potential for chemical segregation, greater difficulty in forging and controlling grain size and the much reduced heating and cooling rates during heat treatment all have potentially significant effects on short and long-term properties. In addition to development and long-term characterisation of the base alloys, development of welding procedures is also necessary, especially for casing materials. Thus development of welding consumables is essential together with long term characterisation of the weld metal and of welded joints. To date the development of high temperature alloys has been dominated by the need for improved creep strength. However, as target temperatures increase, the possibility that steam oxidation will limit application temperatures becomes significant. Current development programmes are now investigating oxidation resistance and it is already clear that some alloys are limited by their oxidation resistance. Attempts to design alloys to improve this characteristic have not so far been successful but an alternative approach is the development of coatings for protection against steam oxidation. This may lead to a radical rethink in alloy design as the requirement for inherent oxidation resistance of the alloy is reduced. Coatings are required that retain their integrity for service lifetimes of at least h. Thus not only must the primary properties (oxidation- or erosionresistance) be sufficient for this but, in addition, the coating must not be susceptible to cracking, spallation or degradation through interaction with the substrate. Page 10 of 11

11 4.2.2 Low temperature Development of materials for single-cylinder turbines and the LP turbine is also required (and often neglected). Rotor development is likely to focus on tailoring properties to site-specific requirements eg high strength where needed and improved SCC resistance where critical. Probable approaches are multi-segment welded rotors, local heat treatment or graded composition rotors. The continued trend towards longer last stage blades requires higher strength materials. This needs to be coupled with improved fracture toughness and SCC resistance. Evaluating the resistance of materials to stress corrosion crack initiation under realistic conditions requires testing in excess of 5 years duration. Test acceleration through using more aggressive test environments is possible but not completely reliable. Improved understanding of the SCC process coupled with predictive modeling is required to develop a robust route to enable the test duration to be reduced Underpinning Activities In addition to the research required to develop new materials there is also a need for underpinning work to develop inexpensive and relevant test facilities to evaluate the material developments. Specific needs are: Non-invasive inspection techniques for in situ assessment of material condition Improved understanding of tests on service-exposed material to predict remaining lifetime Test procedures for miniaturised specimens that can be directly correlated with tests results from standard sized specimens Facilities for extended duration exposures to environments that reproduce service conditions eg high pressure steam, solid particle erosion etc Page 11 of 11