POWER-GENEUROPECONFERENCE2014 Construction,CommissioningandOperationofthe 800MWUltraSuperCriticalCoal BiomassFiredPowerPlantofGDFSUEZinRotterdam Authors: Philippe Bachez (Tractebel Engineering) Peter Vyvey (GDF SUEZ Energie Nederland) Frank Peeters (GDF SUEZ Energie Nederland)
Abstract On July 2013, the new 800 MW super critical coal biomass fired power plant, owned by GDF SUEZ and located on the Maasvlakte in the Port of Rotterdam, was first fired. This paper describes how GDF SUEZ dealt with the risks brought along with the use of T24 in the consecutive phases of construction, commissioning and operating the new power plant. As soon as the first problems with T24 arose, GDF SUEZ and Tractebel Engineering immediately decided to improve significantly the control of the quality during the manufacturing and the erection on the Rotterdam site through Q&C measures. The number of onsite welds was reduced with the agreement of the boiler designer. Research programs were initiated and after a profound examination of the potential root causes of the T24 problem, mainly stress corrosion cracking, some additional provisions were agreed with the supplier to face the specific aspects related to the presence of steel grade T24 7CrMoVTiB10-10 in the boiler and so continue with the initial T24 material selection for the Rotterdam unit. After finishing the construction of the boiler and before first firing, a few measures were implemented in the commissioning phase of the project such as the realisation of a Stress Relief Heat Treatment with hot air, the modification of the boiler acid cleaning, the implementation of oxygen concentration measures & control and the adaptation of the temperature profile during operation. The paper will also describe how the operations and maintenance are being organized in order to deal with the special attention this T24 boiler will require during his complete life cycle and the organisation of a systematic survey of the material behaviour during the first year of operation. Furthermore, the first results and the performance of the power plant during its first year of firing will be described.
TheGDFSUEZRotterdamCoalBiomassFiredPowerPlant By end of the year 2006, the GDF SUEZ group decided to invest in new high efficiency 800 MW ultra supercritical unit Coal Biomass Fired Power Plant (CFPP) for Rotterdam (The Netherlands). In May 2007, a Power Block, including the main steam generator with its downstream flue gas treatment units (DeNox and DeSox), the steam turbine and generator train and the associated balance of plant was contracted to Hitachi Power Europe (HPE). The Power Block was part of a multi lots procurement approach elaborated by GDF SUEZ Energie Nederland with the support of Tractebel Engineering as Owner s Engineer. The main technical features of the coal biomass fired power plant located in Maasvlakte Rotterdam are summarized as follows: Gross power output (BMCR) : 790 MWe Net efficiency (LHV) : 46% Steam parameters at turbine inlets o High pressure live steam : 566 kg/s at 260bar and 600 C o Hot reheat : 451 kg/s at 61bar and 620 C Cooling system : once through open cooling Combustibles : coals within a range between 22 to 29 MJ/kg unit designed for coal / biomass wood pellets co-combustion Flue Gas Emissions o NOx at 6%O2, dry ref conditions : max 50 mg /Nm³ o Dust at 6%O2, dry ref conditions : max 3-5 mg /Nm³ o SOx at 6%O2, dry ref conditions : max 40 mg /Nm³ T24featuresissues The boiler tube material 7CrMoVTiB 10-10 also called T24 (American denomination) was developed by mid-1990s by Vallourec & Mannesmann. T24 was presented as a material not requiring a Post Weld Heat Treatment (PWHT) and responding to the need to have high yield strength and creep rupture strength imposed by the increase of temperature and pressure of high efficiency ultra supercritical plants ([1]). T24 material was then selected by the boiler designers to replace the traditional 13CrMo4-5 (also called T12) used for the membrane walls of numerous CFPP s.
The following figures summarize the T24 composition and creep rupture strength evolution: Figure 1 & 2: T24 composition and average creep rupture strength (sources HPE [2]) As shown in these figures, T24 material can be qualified as an advanced material presenting a quite complex composition and showing enhanced creep strength resistance almost two times higher than the one of T12 in a temperature range between 500 C and 520 C. In the Rotterdam CFPP boiler design, T24 has been selected for the membrane walls of the boiler furnace evaporator section, for the heat exchangers supports of the convective section and in the upper portion of the first super-heater section (SH1) of the platen type (see figures 3 and 4).
Figure 3: boiler furnace & evaporator section Figure 4: Heat exchangers convective section Since 2009, 17 of the newest generation USC CFPP units are being built or commissioned in Europe (see figure1 of [1]). As from March 2010, it became clear that a large majority of these new plants would be confronted with a major risk of excessive boiler leakages associated to the use of T24 material in the boiler water membrane walls. Numerous leakages in T24 weld seams occurred first during the commissioning of the Steag s Walsum 10 and Vattenfall s Boxberg R units. The damages were located in boiler membrane walls area where water flows in two phase mixture.
GDFSUEZriskassessmentandmitigationstrategy Confronted with the T24 issue, GDF SUEZ decided the creation a dedicated T24 Task Force gathering the competences of different entities of the Group and having as major objectives to analyse the issue, identify the potential root causes, study the possible alternatives and assess the risks brought along with the use of T24 in the consecutive phases of construction, commissioning and operating the new power plants. PartI:Research,Q&CmeasuresandriskmitigationstrategiesforT24 duringconstruction As soon as the first problems with T24 arose, GDF SUEZ and Tractebel Engineering immediately decided to improve significantly the control of the quality during the manufacturing and the erection on the Rotterdam site through Q&C measures. To better manage the level of stresses induced during membrane walls construction, the following actions were adopted with the boiler designer HPE : HPE decision to manufacture the membrane walls in an HPE s subsidiary company in Germany ; selection of a well trained erection company having recent experience with T24 membrane walls welding works ; detail review of the specific Welding Procedure Specifications (WPS) by focusing on the narrow processing window of parameters such as preheating temperature, inter-pass temperature, welding speed, welding energy (current & voltage), ; increasing of the quality control and qualification of T24 welders : o welders were selected and trained to increase their skills ; o organization of systematic NDT with strict rejection criterion, crater cracks focus, o permanence presence of GDF SUEZ in the workshop and on site maximization of the workshop welds by increasing the length of the manufactured individual vertical panels ; detail review of the erection procedures with focus on : o the preparation works (humidity, temperature, handling, ) ; o the erection sequence to minimize the panels deformations ; o the gaps control before final welding. Simultaneously, members of the T24 task force took part at Experts Groups composed of boiler designers, users and scientists. Research programs were initiated.
The sensitivity of T24 to Stress Corrosion Cracking was established and two mechanisms were identified: the Hydrogen induced stress corrosion cracking (HI-SCC) as reported in the VGB publication [1] and the anodic stress corrosion cracking as presented by HPE in Power Gen Europe 2013 [3] and published in Modern Power Systems [4] The following figure 5 presented in the VGB publication [2] summarizes the very complex mechanism associated to HI-SCC phenomenon i.e. the fact that cracks are initiated with the simultaneous coexistence in the material or its environment of : a minimum stress level ; a medium at the origin of the Hydrogen production ; a material structure acting as a catalyst. Figure 5: HI-SCC mechanism (source VGB [2]) As outcome of HPE research programs presented in [3], the anodic stress corrosion cracking mechanism was also presented as root cause of the T24 issue. This corrosion mechanism is similar to the one of the HI-SCC one but is initiated by an elevated oxygen level in the feed water (> 150 ppb) and is most pronounced in a temperature window between 180 C and 220 C. To eliminate the risk and provided it is feasible in terms of pressure and temperature features, the ultimate radical solution is to replace the T24 material by another material insensible to SCC. This solution has been adopted for different units confronted with the T24 problematic. As alternative to the material change, which has by the way a huge impact in terms of time delay, mitigation measures acting both to reduce the influence of the stress level and of the medium through the process can be envisaged. For the Rotterdam unit, given the anticipated measures taken during the boiler construction and based on the first results reached by RWE for the BoA 2&3 projects ([2]), additional provisions were agreed with HPE to face the specific aspects related to the presence of steel grade T24 7CrMoVTiB10-10 in the boiler and so continue with the initial T24 material selection.
PartII:DealingwithT24duringcommissioningandstart-upoftheplant After finishing the construction of the boiler and before first firing a few measures were implemented in the commissioning phase of the project. StressReliefHeatTreatment(SRHT) The first of the pre-commissioning measures was the realization of a Stress Relieve Heat Treatment (SRHT). The purpose of this measure was to reduce the unavoidable level of stresses induced during the membrane walls construction. It consisted in the heat treatment of the complete boiler T24 pressure parts assembly by means of compressed hot air heated from temporary air burners installed around the furnace. The figure 6 gives a schematic overview of the SHRT process carried out in the Rotterdam unit and was presented by HPE during the Power-Gen Europe 2013 exhibition (see [3]). As explained in HPE publication [3], the target of the SRHT is to reach a constant temperature of about 500 C during a sufficient period of time to induce a stress relaxation in the highly loaded area of the membrane walls. Figure 6: schematic overview of SRHT (source HPE [3]) For the GDF SUEZ Rotterdam unit, HPE has targeted to reach a mean temperature of 520 C during 48 hours.
The realisation of this pre-commissioning operation has required: the installation of 24 oil burners (hot air guns) in the furnace (consumption of ~ 350 m³ of oil) ; four temporary openings in the non-pressure parts boiler roof ; a closing plate at boiler outlet (downstream economiser) ; 28 air compressors ( 50 C/ 170 C, 6 bar) to feed the boiler ; the use of temporary acid cleaning piping to convey air from the air compressors area to the boiler During this operation, in addition to the Health and Safety aspects, GDF SUEZ has particularly focused its attention on the following key points: a complete verification of pressure parts has been requested : in particular the compliance of the design features with the uniform temperature level induced during the SHRT operation was assessed for some material of boiler piping; the thermal expansion of the boiler has been continuously monitored from the start of the firing to verify that no additional stress was induced due to differential expansion in between the furnace side walls ; verification that no flue gas leak was susceptible to increase the temperature at air preheater inlet (LUVO) Adaptationoftheboilercleaning A second measure has consisted in the modification of the boiler acid cleaning. Further to the first damages caused by HI-SCC at the Walsum 10 unit (hydrogen produced by the decomposition of the inhibitor used - [3] and [4]), HPE decided not to further acid clean the parts of the boiler containing T24 material. Therefore the complete evaporator and super-heater SH1 sections were flushed, degreased, by-passed during the hydrofluoric acid cleaning phase and finally passivated. To assess the cleanliness of the evaporator it was proceeded to the control of the particles caught by different temporary strainer meshes sizes installed upstream the evaporator recirculation pump. A measurement of the flow on each individual evaporator tube was also performed before first firing. Fine strainers installed in the steam turbine admissions valves have been used as from the first firing and have been removed at the end of the hot commissioning period i.e. after BMCR load has been reached for a sufficient period of time.
Oxygenconcentrationandtemperaturewindow To act on the influence of the medium circle part of the anodic stress corrosion mechanism identified by HPE, the VGB-R 450 Guidelines were adapted by HPE so that following targeted oxygen concentrations have to be met: O2 concentration < 10 ug/kg in All Volatile Treatment (AVT) and O2 concentration < 70 ug/kg in Oxygenated Treatment. Continuous oxygen measurements were implemented on the feed water and the boiler water. These measurements are part of the automatic monitoring and control system taking into account the combination of O2 content and boiler evaporator water temperature for AVT and OT treatments. In case of non respect of the thresholds, a boiler trip is initiated automatically. To reduce the risk of coexistence oxygen content within a critical temperature window : the number of O2 injection points were reduced : the O2 dosing downstream the feed water tank has been permanently blocked so that in OT mode, the oxygen injection is only realized in condensate upstream the feed water tank ; the start-up curves were adapted to ensure a fast smooth passing of temperature window between 150 C 280 C : practically this is implemented by tuning the burners load & pressure gradient during commissioning to target a start-up rate of 3.5 K/min within the temperature window. For cold start-up, the inertisation of the economiser with N 2 prior to filling the Water / Steam System with a controlled O2 content < 20 ug/kg has been systematically implemented with the purpose to ensure the absence of oxygen during restart: economizer acts as a N2 buffer. HI-SCC T24 being sensitive to HI-SCC, the unavoidable conditions under which hydrogen is produced during the building of the magnetite layer (Schickorr reaction) were closely controlled so as to reach a sufficient stable and dense magnetite layer as soon as possible. In that respect, just after the first firing, the boiler has been operated in steam turbine by-pass mode during 3 to 5 days to control the building of the magnetite layer in the evaporator by means of the monitoring of the evolution (i.e. decrease and stabilisation) of: the iron concentrations in the condensate, the feed water, the boiler water (circulation mode) and the live steam and the hydrogen concentration in the live steam.
PartIII:Firstperformanceresultsduringcommissioning At the time of writing this paper i.e. April 2014, the steam generator of the CFPP Rotterdam Unit has been fired during approximately 2000 hours and no leaks on the T24 portions have been detected. Although this first indication seems to reveal that the T24 installed in the GDF SUEZ Rotterdam unit has satisfactorily passed the commissioning period, it is too early to conclude that the effect of the HPE mitigations measures have ensured the long term reliability of the T24. The operation and maintenance services of GDF SUEZ Energie Nederland are being organized in order to deal with the special attention this T24 boiler will require during his complete life cycle. Accordingly, GDF SUEZ Energie Nederland with the support of Tractebel Engineering and Laborelec have decided to implement and adopt the following mitigation measures during the operation : implementation of a strict monitoring of the feed water quality with automatic actions programmed in the DCS ; anticipation of dedicated T24 repair procedures established with HPE and taking into account : o the location of the repair and its accessibility ; o the size and the type of the defect ; o the number of defects i.e. cracks. organisation of a systematic survey of the material behaviour during the first years of operation : series of sensible welds to be examined were identified with HPE and two periods of examination were arrested : o during first legal inspection o within the 2 years during the contractual defect liability period
Conclusions The problem associated with the use of 7CrMoVTiB 10-10 (also called T24) for the membrane walls of CFPP boilers has concerned major electrical producers in Europe amongst which the group GDF SUEZ. GDF SUEZ has assigned the risk assessment to stay with T24 material or to find alternative solutions to a dedicated T24 Task Force gathering the competences of different entities of the Group. The mechanism of Stress Corrosion Cracking has been identified as the root cause of the damages observed in the different units in commissioning in 2010 2011. For its Rotterdam unit, after having immediately decided to improve significantly the control of the quality during the construction, GDF SUEZ has opted, based on the investigations of the Task Force and further to the return of experience of other similar units, to let HPE implement different mitigation measures and so stay with the original T24 material configuration. To reduce the risk of Stress Corrosion Cracking, the T24 Mitigation Measures have consisted in a Stress Relief Heat Treatment, in the by-pass of the T24 boiler portion during the acid cleaning, in the modification of the feed water quality requirements and in the adaptation of the start-up temperature profile. Although no leaks on the T24 portions have been detected after approximately 2000 hours of firing, it is still too early to conclude that the effect of the HPE mitigations measures have definitely ensured the long term reliability of the T24. Accordingly, GDF SUEZ Energie Nederland is performing a strict control of the parameters susceptible to have an influence on the boiler parts in T24 and is being organized to carry out a systematic survey of the material behaviour during the first years of operation.
References: [1] : Quality management at RWE using T24 boiler material as an example VGB Power Tech English version of a paper published in German in 11/2011 by Dr.-Ing Dipl.-Ing. Ralf Nowack, Dipl.-Ing. Christoph Götte and Dr.-Ing Dipl.-Ing. Simon Heckmann [2] : Hitachi Power Europe Slide presentation to GDF SUEZ- 2006 [3] : HPE paper Modified commissioning procedure for USC boilers using T24 Power Gen Europe 2013, Vienna by Jörg Böve and Martin Becker of Hitachi Power Europe Gmbh [4] : T24 experience: an Hitachi Power Europe perspective Modern Power Systems October 2012