Mat UK Energy Materials Review R&D Priorities for PF Boiler Materials. 7 th August Prepared by: Matt Barrie Doosan Babcock Energy Ltd.

Transcription

1 Mat UK Energy Materials Review R&D Priorities for PF Boiler Materials. 7 th August Prepared by: Matt Barrie Doosan Babcock Energy Ltd. Page 1 of 8

2 1. Executive Summary: This report summarises the current status of material use in PF Power Plant. Future material development requirements for higher efficiency ultra supercritical power plant are identified. The following key recommendations are made for future research programmes:- long-term creep rupture and thermal fatigue on new materials. long term fireside corrosion and steam side oxidation testing in real environments. fireside corrosion testing in oxy-fuel environments in laboratory rigs. development of protective coatings. further development, refinement, integration and validation of microstructural modelling techniques. validation and approval of new nickel alloys further development and validation of life assessment procedures. Page 2 of 8

3 2. BOILERS TECHNOLOGIES. Electricity is a key component of modern life and around 40% of that electricity is generated through the combustion of coal. Pulverised Fuel (PF) combustion is the most widely used method for power generation and this is likely to continue for the foreseeable future. The drive to reduce greenhouse gas emissions and other pollutants caused by the combustion of coal has resulted in the development of a two pronged strategy which can be used together or in isolation. The first, dealt with in a subsequent section, is carbon capture which addresses different methods of removing carbon dioxide from the flue gas. The second is to increase the amount of electricity generated per tonne of CO 2 produced. Hence the need for higher efficiency power plant through higher steam temperatures and pressures with a move from sub critical to supercritical operation. This twin track approach is shown graphically in figure 1. Carbon Dioxide Carbon Reduction -95% TRACK 2 Capture and Storage (CCS) -60% Increased Efficiency,, TRACK 1-23% Baseline - - Possible Now Medium Term Long Term Time Figure 1 This has, in turn, necessitated the development of new materials capable of achieving these advanced conditions. A European initiative begun in 1999 under the project Thermie 700 set itself the ultimate target of a supercritical cycle with a final steam temperature of 700 C. It was recognised that this would have to be achieved incrementally and different targets for the various material groups were set as shown in table 1. Material Type Metal temperature Average stress rupture Ferritic Materials 650 C 100 MPa Austenitic Steels 700 C 100 MPa Nickel Alloys 750 C 100 MPa Table 1. Page 3 of 8

4 The success or otherwise of meeting these targets is discussed in detail for the different applications in the following sections. 3. Boiler Materials 3.1 Furnace Walls. In both the spiral and vertical furnace wall designs, parallel neighbouring tubes are joined by welded membrane and thus weldability and post weld heat treatment (PWHT) considerations become a factor in the choice of materials along with strength, corrosion resistance and cost. In supercritical plant operating in the common regime of 250 bar, 540 C steam, the maximum fluid furnace exit temperature is ~ 420 C. This corresponds to a tube mid-wall temperature some 30K higher at 450 C at the start of life and a further 10k higher later in the life cycle due to the insulating effect caused by the growth and deposition of magnetite. For a design life of 100,000 hours 13CrMo44 (1Cr½Mo) has adequate creep strength for these conditions but this excludes fireside corrosion considerations. All boiler manufacturers offer some form of staged combustion to limit NO x emissions. In all cases this requires substoichiometric combustion in the lower portion of the furnace chamber. Such conditions pose the danger of furnace wall fireside corrosion which can result in rapid thinning and early failure of the membrane wall tubes. Hence some manufacturers prefer to use 10CrMo910 (2¼Cr 1Mo) with its higher chromium level. However significantly higher chromium levels >20% are required before any real improvement in corrosion rate is likely to be experienced. Co-extruded tubing with an austenitic outer layer has been considered but this poses tremendous difficulties due to differential thermal expansion within the furnace and consequential distortion. Consequently all boiler manufacturers offer some means of ensuring more oxidising conditions close to the furnace walls through burner arrangements. Under conditions where this gives inadequate protection consideration can be given to weld overlay or spray coatings particularly around the burners. For the more advanced steam conditions in ultra-supercritical plant of ~325bar and 620 C steam, two new alloys have been developed from the 2¼Cr 1Mo standard. These are 7CrWVNb9-6 (T23) and 7CrMoVTiB10-10 (T24). Both have been designed to avoid the need for PWHT as this can cause difficulties during erection and for in-service repairs. Currently available ferritic alloys with higher chromium levels for improved creep strength and corrosion resistance such as X10CrMo9-1 (T91) and X10CrMoVNb9-2 (T92) do require PWHT and hence are not favoured for Furnace Wall applications. Since, as explained previously, austenitic materials are not considered suitable, the only materials available for the more extreme conditions which might pertain as steam conditions approach 700 C are the nickel alloys and in particular Inconel 617.These alloys are significantly more expensive than their ferritic counterparts and to warrant their use requires a significant improvement in boiler effieciency. The relative creep strengths of the various options are presented in Figure 2. Page 4 of 8

5 T24 Inco 617 Allowable stress (MPa) T23 1Cr ½ Mo T Temperature C (100 khrs) 3.2 Superheater Tubes Figure 2 : Candidate Furnace Wall Materials Superheater tubes are designed to operate at temperatures some 35K (for convection heated) to 50K (for radiant heated) above the live steam temperature. The current state of the art ferritic alloys T91 and T92 are suitable for metal temperatures up to ~615 C and therefore have their part to play in these advanced designs. Unfortunately the higher chromium versions such as T122 and VM12 have recently been shown to suffer from the precipitation of Z phase with time which considerably reduced their creep strength a reduction of ~30% having been applied by ASME to T122, resulting in allowable stresses not much better than T22. However, in addition to stress rupture and steam side oxidation considerations, the influence of the coal on fireside corrosion has to be taken into account. For example in the 70 s CEGB built a plant with final steam temperatures of 565 C and burning coals with chlorine contents around 0.15%. For this, the ferritic alloys were shown to be inadequate and re-tubing with austenitic steels of type 316 and type 347 was required. There have since been further improvements with these 18/8 type austenitics by the addition of various alloying elements such as copper, boron and nitrogen which have further improved their stress rupture capabilities resulting in grades such as Super 304H. Modified thermomechanical treatments have also been used to limit grain size and hence increase the potential for chromium diffusion along the grain boundaries to enhance the corrosion resistance as demonstrated with type 347HFG. Another successful approach has been to use shot peening of the tube bore to aid the diffusion of chromium during operation and significantly reduce the rate of bore oxidation. Nevertheless it is considered that the 18% chromium in these steels will limit their application to ~620 C. For higher metal temperatures several austenitic alloys are available including NF709 and type 310HNbN (HR3C) but a recent development by Sandvik under the Thermie 700 project, designated Sanicro 25, has been shown to have superior stress rupture properties matching the target for austenitic alloys of 100MPa average stress rupture at 700 C for 100 khrs. The next step beyond this is again to move to the family of nickel alloys. Inconel 617 is a good candidate as it has a proven history, in particular in German plant but it was recognised in the Thermie 700 programme that it was incapable of achieving the optimum target stress rupture requirement for nickel alloys of 100MPa average stress rupture at 700 C for 100khrs. Page 5 of 8

6 An alloy developed by Special Metals, designated Inconel 740 did, however, meet this target. This alloy is a variation on the existing alloy Nimonic 263 but with modifications such as increased chromium and niobium with reduced molybdenum to improve fireside corrosion resistance. Being a precipitation hardened alloy it requires a more complex thermal treatment than the solution strengthened 617 involving solution treatment and precipitation hardening during fabrication. This is of particular significance both to tube bending operations and developing the full strength in weldments. Although laboratory testing has been carried out for all of these new alloys this does not always translate into boiler operation hence in plant testing is required to validate their use. The recent component test facility COMTES 700, installed in E.ON s Scholven plant included test loops in Inconel 617, Sanicro 25 and Inconel 740 which will provide valuable information on their performance. Also the fireside corrosion effects from alternative low carbon technologies such as oxy-fuel firing will undoubtedly introduce new problems for which other alloys, intermediate between austenitic and the high nickel alloys, such as X7NiCrCeNb32-27 (AC66) might prove valuable. A considerable amount of work is envisaged in investigating and validating all of the available materials for this new technology. 3.3 Steam Separating Vessels For once-through, supercritical units, steam separator vessels are often the heaviest walled components in the boiler circuit and are subject to severe thermal fatigue stresses. They separate the saturated steam, which flows on to the superheaters, from the water which is returned to the boiler water feed train. Materials such P11 have been used satisfactorily in the past. To minimise wall thickness and hence maximise flexibility, materials with higher yield strength are preferred such as 15NiCuMoNb (WB36) or P Headers and Steam Pipes. Headers and pipework are situated outside the furnace and so fireside corrosion is not a factor in material selection, however steam oxidation of the bore must still be considered. Early supercritical plant operating around 540 C to 560 C used X20CrMoV121, a nominally 12% Cr martensitic steel, for the highest temperature headers and pipework. The development of P91 modified 9% chrome, enabled higher pressures and temperatures to be accommodated. However the level of chromium limited their application, due to steam oxidation considerations, to ~600 C maximum. With the addition primarily of tungsten to produce NF616 (P92) and E911(P911), a moderate increase in metal temperatures to ~610 C was possible. Attempts were made to produce higher chromium variants such as HCM12A (P112), NF12 and VM12 with around 11 to 12%Cr and early stress rupture data was promising. However at longer times a significant reduction in stress rupture strength was noted due to the formation of a complex nitride precipitate Cr(Nb,V)N called Z-phase. It seems that higher chromium levels and operating temperatures increase the likelihood of its formation. Consequently it seems likely that to move to higher steam temperatures using a monolithic pipe will mean changing to the much more expensive nickel alloys. Another concern with the 9%Cr steels is the potential for weldments to fail at the outer edge of the heat affected zone known as the type IV position. Work is currently underway using higher boron levels in these steels as means to restrict or eliminate the phenomenon though the exact mechanism by which this happens is unknown. As part of the AD700 programme, the first thick walled pipe in Nimonic 263 at 310mm OD x 66mm wall was produced with weldability and stress rupture testing to date providing promising results(see Figure 3). Further work is on going to allow the material, in this form, to be used in boiler manufacture. However, as it is much more expensive than its ferritic Page 6 of 8

7 counterparts, significant improvements in efficiency need to be realised to make it economically viable. Figure 3 Nimonic 263 extrusion One method of increasing the service temperature of the ferritic alloys being considered is the application of an integral oxidation resistant conversion coating on the bore of the pipe. If this is successful it is hoped that steam temperatures towards 650 C will be possible. 4. Future R & D Needs. Over the past 25 years considerable effort from the materials community has been channelled towards developing alloys for the higher temperatures and pressures required for ultra-supercritical boiler plant. The main driver has been to improve efficiency and hence reduce the emission of greenhouse gases and pollutants per kilowatt of energy produced. European and UK supported initiatives, coupled with Japanese developments have resulted in significant improvements in the steel grades available for power generation. Nevertheless the barrier to ferritic steels being capable of application at temperatures above 600/610 C has yet to be breached. Much of the available data supporting these new and developing materials is from laboratory based short term creep rupture and corrosion studies with little opportunity to test them in real environments. The recent test loops at Scholven are an example of a major attempt to rectify this shortcoming. Fireside corrosion studies are required specifically for the new oxy-fuel technologies with all of the candidate tube materials. These will, of necessity, need to be comprehensive laboratory based tests as real environments do not exist and large scale burner test facilities are cost prohibitive. Alloy assessment and development through computer based programmes such as Thermocalc and MT-data continues with the models being further refined by the inclusion of new data as it becomes available. There are also programmes to integrate various modelling techniques at the micro, meso and macro scale to predict the effects of alloy composition Page 7 of 8

8 and thermal history on the thermodynamic and diffusion controlled processes which affect the long term creep life and damage accumulation in ferritic and nickel alloys. The development of coatings or surface treatments that can confer substantial resistance to both fireside and steam side corrosion continues to be an area where much work is needed. This may be the only route available to take ferritic/martensitic steels towards their ultimate target of 650 C for header and pipework applications. In conjunction with this, however, improvements in the alloys themselves are considered necessary to take account of type IV behaviour at welds or design rules with appropriate weld reduction factors will need to be agreed. Finally, nickel alloys are considered the only realistic solution for final steam temperatures around 700 C. Validation and approval of alloys Inconel 740 and Nimonic 263 are considered to be key to success in this area. In summary. long-term creep rupture and thermal fatigue on new materials. long term fireside corrosion and steam side oxidation testing in real environments. fireside corrosion testing in oxy-fuel environments in laboratory rigs. development of protective coatings. further development, refinement, integration and validation of microstructural modelling techniques. validation and approval of new nickel alloys further development and validation of life assessment procedures. Page 8 of 8