International Conference Power Plants 2012

Transcription

1 International Conference Power Plants 2012 Society of Thermal Engineers of Serbia Oct. 30th - Nov. 2nd, Zlatibor, Serbia Abstract In the Balkan and Central Europe, there is a large fleet of older oil & coal fired power plants with a potential for repowering into modern combined cycle power plants. This paper will review the specific HRSG design features required for such repowering projects. Compared to green field projects, repowering a steam turbine with a new train of gas turbine and HRSG includes some special challenges. The HRSGs have to be tailor made to suit the existing site constraints. We will explain how a Vertical type HRSG can be adapted and modelled to match a limited foot print. In addition we will present the specificities of the design of HRSGs when the Gas Turbine fires heavy fuel oil, as mandatory in some projects. CMI has recently completed such a large repowering projects, for instance Senoko Singapore; Dunamenti Hungary, Shoiaba/SEC/ Saoudi Arabia, These experiences will be the support throughout this paper to explain these specific repowering challenges. Fig.1 CMI is a HRSG specialist and offers both Horizontal and Vertical designs

2 Background of Senoko Project In December 2004, Senoko Power of Singapore has completed an innovative plant repowering project. Three old large coal fired boilers had been demolished to be replaced by 3 gas turbines of 250 MW each, coupled with a Heat Recovery Steam Generator (HRSG) based on state-of-the- art 3 pressures plus reheat. Produced steam is used in the former old steam turbine and condenser. This is the repowering concept of an old steam turbine, which has typically a longer life span than the boielrs. This paper reviews all of the specific characteristics of the HRSG for repowering as used for Senoko project but also at Dunamenti a repowering project in Hungary. Compared to a greenfield plant, where standard reference plant concepts can apply, repowering projects must always be specific to suit existing plant layout and steam turbine operating conditions. This was especially true at the Senoko plant. Firstly, the new HRSGs had to match the very limited foot print of the old conventional boilers, where dimensions of the new HRSG had to be adapted. Then, the HRSG design was selected as Vertical type. Secondly, modular construction including highly prefabricated heat exchangers were required to match the 18 months stoppage. Finally, the Senoko facility is a very congested site with no room for large cranes. Consequently, hydraulic lifting jacks have been used instead for boiler erection of heavy items. A good deal of existing civil structures and cooling systems were re-used. After the first GT26 block, having the equivalent capacity of more than two of the smaller steam sets, has come into operation, it was possible to shut down the remaining two units for repowering. The second repowering step was then performed in a narrow 18 month window. Thanks to this two-steps repowering, the interruption time was minimal and the plant returned to service as scheduled at the end of Alstom was the project leader, and they in turn awarded the HRSG to CMI. The choice of CMI, as a specialist in the Vertical HRSG, was significant. The HRSGs are Vertical natural circulation with three pressure levels plus reheat : HP, 322 t/h at bara, 568 C; IP, 27.9 t/h at 41.5 bara, 320 C; LP 17.8 t/h saturated at 5.4 bara, 237 C and RH, 332 t/h at 39.4 bar 568 C.

3 Site limited available area At Senoko, there were constraints imposed by the space occupied by the original fired boiler which had to contain the gas turbine, the HRSG and the connections to the steam turbine. So, available area per HRSG was limited to only 30.6 meters in length by 28.1 meters in width. HRSG with all auxiliaries (feedwater tank, feedwater pumps, sampling, dosing skid, ) had to be included in such a reduced area. Inside the building, a cargo lift was also to be installed. All piping had to remain within this perimeter boundary because of the surrounding enclosure, and some limits of supply and even Alstom s equipment was located inside the HRSG enclosure creating major risks of interfaces. Last but not least, the GT outlet extended about 1 meter into the boiler enclosure, reducing even further the available area for HRSG. Therefore, it was challenging to install such a large HRSG in such a reduced area. At the early engineering stage, CMI exchanged with Alstom 3D model of HRSG including auxiliaries. Alstom then consolidated its 3D plant model with PDMS software. Detailed three dimension plant modelling prevented clashes at the engineering stage without adverse construction problem (Fig.2). Fig.2 HRSG modeling in 3 dimensions

4 Vertical HRSG flexibility Standard Vertical HRSG for such GT class F combined cycle typically measures an overall length of about 35 meters. This length is based on 20.4 meters tubes long which are the longest tubes for Vertical HRSG used by CMI without any other limitations. Such standard design applies for greenfield site or when there is no space limitation. Even though this standard boiler is already quite short, this was not short enough to match the Senoko repowering site. Unlike horizontal HRSG, boiler tube length impacts directly the boiler length; this is a specific feature of Vertical HRSG. CMI took advantage of this lateral and vertical space flexibility arrangement offered by the Vertical design (Fig.3). Typical tubes length 20.4 m GT Typical heigth 9 m Overall boiler length 35 meters Fig. 3 Vertical HRSG overall length based on longest used boiler tubes This feature is very useful in case of repowering. As often, the boiler width was not such an issue in comparison with boiler length. For Senoko, the longest possible tube was 18.4 meters and the boiler casing was enlarged in proportion. While making such casing adjustment to available space, the driving criterion is to keep the gas pressure drop unchanged. In other words, gas velocity and gas path cross section shall also remain unchanged (Fig.4). It is important to remember here that HRSG gas pressure drop is

5 proportional to the square of gas velocity, meaning that gas pressure drop (guaranteed value by HRSG supplier) is very sensitive to available gas flow cross section. Longest tube 20.4 m Reduced tube/boiler length Gas cross section Flexible Gas cross section unchanged Slightly enlarged boiler width Fig. 4 Arrangement flexibility offered with shorter tubes. As a consequence of this wider casing, path had been divided in 3 wide sections instead of 2 as usual for the standard solution. There were 3 heat exchanger modules side by side over 4 levels for a total of 12 modules (Fig.5). The modules were completely shop prefabricated and hydrostatically tested in workshop. The largest module weighted 145 tonnes with overall dimensions 23.6 m * 3.9 m * 2.9m high. They were transported from harbour to site on hydraulic trailers. Heating surface modules flexible arrangement Gas flow Standard 'greenfield' arrangement Senoko enlarged casing width Fig. 5 Heat exchanger modules arranged in 2 or 3 modules in width This way, it became possible to fit the HRSG on this site. In case of available length is even shorter, a last option would be by means of entering flue gas on the large casing face instead of the standard small face (Fig. 4). This is a specific feature of the Vertical HRSG as the first

6 heating surface is about 9 meters high (Fig.3), and this allows sufficient space for the inlet duct arrangement underneath. This option is applicable up to middle size HRSG and is useful in case of repowering plant where, for instance, the existing steel structure shall be recovered. But, this option was not requested at Senoko. Boiler modular construction Heat exchangers modules were factory assembled under strict CMI quality control. As per CMI standard design, each module was made of parallel serpentine tubes mounted on tube support plates and connected to headers at each ends. As a result of this module prefabrication design, only header to header welds had to be carried out at site. An important constraint at Senoko site was to ensure that there would be enough access around the operating unit for the repowering of the second and third units. The hydraulic trailers carrying the modules had to manoeuvre around the operating unit of phase 1 (Fig. 6). To perform so precision movement, trailers selected were self-propelled rather than tractor pulling. This allowed more flexibility in those manoeuvres because of the reduced convoy length, and its large angle orientation wheels. Also, those advantages offered by the self-propelled trailer were required for the final positioning of modules inside boiler frame due to limited space left as explained hereafter. Those hydraulic trailers proved to be very accurate in this exercise with a positioning tolerance of only 1 or 2 mm. Taking into account maximum trailer turning radius, a cinematic study of movement sequence had been performed by CMI during engineering.

7 Fig. 6 Boiler modules on hydraulic trailor had to manoeuvre on congested site Module lifting with jacks Senoko site precluded the use of a large crane. In case of prefabricated modules for Horizontal HRSG, large cranes are typically needed to tilt and lift them. Instead for Senoko, module lifting was performed with 28 hydraulic jacks installed on top of steel structure. This module erection procedure is standard for CMI Vertical HRSG. The first module was brought to position in the frame and its 7 tube support plates were attached to those jacks through suspension cables and plates. The hydraulic trailer was then lowered, transferring module weight carried to those cables. The process was repeated for the other two modules to complete this first level. This completed level was then jacked up sufficiently for the next level of modules to be placed underneath it similarly, and the process was repeated until all 4 levels of modules had been suspended (Fig. 8). It is important to note that modules remained always in horizontal position; module tilting operation is not required for Vertical HRSG erection. The complete assembly was then jacked up to its final position in the steel structure. At that stage, the complete pressure parts weight amounting to 1450 tonnes was still hang on those 28 jacks. By introducing pins in suspensions plates and lowering jacks, weight was

8 transferred to the 28 final suspensions points. Hydraulic jacks were released, dismantled and reinstalled on the next unit. Then, adjacent header ends were welded together; no other internal pressure part welding was needed except for a few tube/tube welds between level 1 and level 2. From modules arrival harbour up to final suspension of the 12 modules on to the boiler frame, it took only 6 days without using crane, nor even scaffolding. This is a remarkable short time also due to this work repeat. Fig. 7 Second level of module ready to be lifted Fig. 8 Completion of 12 modules suspended Typically, hydraulic jacks are better synchronized in jacking up than lowering loads. Lowering load is a feature that is not normally needed by CMI module erection procedure described hereabove. However, at Senoko, space was so limited (Fig. 10) that this special feature was required. Indeed, when the erection of each module is in a gradually reducing area, the problem is to get the third module exactly aligned relative to the 2 aside modules already suspended. Hydraulic jacking system had been selected to get the capability to lift and to lower the complete load in a synchronized way. Synchronization between jacks is very critical during the lowering down because of the risk of unbalanced pressure between jacks and consequently uneven load distribution on the steel structure. Considering the 1450 tonnes hanging on this jacking system, uncontrolled load distribution between the 28 jacks would have been unacceptable. To give clearance to the trailer to manoeuvre for the third module to be positioned exactly underneath, the whole assembly was jacked up of several meters and then relowered at the initial elevation to install suspensions plates. Hydraulic jacking system

9 proved to be very accurate in positioning with a maximum tolerance of only one of two millimetres (Fig.9). Fig. 9 Module erection Fig. 10 Self propelled trailer manoeuvres in limited space Indoor HRSG Indoor HRSG was specified indoor by Senoko Power. Not only is the Singapore climate characterized by temperatures in the low to mid 30 s and high relative humidity for much of the year, but the power plant is located on the coast and buildings provide protection against airborne salt. The main purpose was weather protection, but enclosure was also used for noise abatement in terms of far field acoustical emissions. It is to be noted here that boiler stack was also equipped with a flap damper for inside weather protection, and boiler bottling up to keep it warm during outage. Compared to Horizontal HRSG, stack damper is a standard feature of CMI Vertical HRSG as the stack is centred on HRSG. For enclosure support, the main HRSG steel structure was easily extended to a secondary frame wrapped and attached all around. Senoko Power had specified to CMI architects some colours and aesthetic criteria (Fig. 11). For instance, all building side walls were extended 2 meters above roof platforms in order to hide top equipment such as silencers and louvers. Those louvers were installed for the natural ventilation of building.

10 Fig. 11 Senoko repowering HRSG buildings phase 1 (CCP3) on left, and phase 2 (CCP4&5) on rigth Dunamenti, Hungary Fig. 12 Dunamentia G3 (GDF-SUEZ ELECTRABEL) BUDAPEST, Hungary

11 After the G2 repowering project in 1995, CMI supplied a HRSG for the G3 project that was commissioned in Fig. 13 Shoiaba/SEC/ Saoudi Arabia, General Arrangement Drawing SHOIABA HRSG project summarized: 10 vertical HRSGs, outdoor Flue gas by-pass stack (as option) Crude oil firing with sootblowers Hot casing (external rockwoll insulation) 2P (LP steam for the external deaerating FWT) Natural circulation Site conditions: 9 to 50 C ASME design HRSG prefabricated in 4 pressure part modules Number of tubes: 3456 tubes of 17 meters Stack

12 Specificities of a HRSG designed for continuous fuel oil firing in the Gas Turbine: Heavy duty gas turbines operate typically on natural gas, without risk of fouling and typical dew point at 60 C Both Horizontal or Vertical HRSG are suitable such purpose Crude oil has high sulfur content with Acid Dew Point around 145 C High sulfur content dictates the proper HRSG selection because all metallic surfaces must remain above ADP to prevent internal corrosion Temperature of condensate water entering finned tubes must be controlled to remain above ADP to avoid acid formation on tubes It limits the heat recovery in the back end of the HRSG Ducting metal must remain above ADP which is not feasible with internal insulation. HRSG must be externally insulated with hot ducting walls at gas temperature (no condensation occurrence) For HRSGs operated on continuous crude oil, heat exchangers are designed as follow: Finned tubes with maximum ~ 160 fins per meter Solid fins prefered Staggered or inline tubes arrangement On line cleaning Sootblowers inside tube banks Off line Water washing system capability and drains Limitation of tube rows per bank for efficient cleaning Good accessibility of pressure parts for inspection Fig. 14 CMI has designed many HRSGs behind gas turbines firing light and heavy fuel oil.

13 Conclusions There is real potential for repowering of old steam turbines in the Balkans. In a lot of old conventional plants, fired boilers have exhausted their useful life before its steam turbine. Over the years, much has been said about repowering, but very little has been done so far. Repowering of those old steam sets into efficient combined cycle with new GTs and HRSG is a cost effective solution. Today, available gas turbines can provide the exhaust energy for steam turbines of MW of which there are many examples in Europe dating back from the 1970 s. These units could be repowered so as to increase power supply with a significant improvement in operating efficiency, flexibility and emissions. CMI has completed the Senoko and Dunamenti plants, which are a very successful example of such repowering. At Senoko, the Vertical HRSG has been proofed to be very accommodating for those the specific repowering constraints, which always require a tailor made design to suit limited space. In addition CMI Vertical HRSGs are uniquely fit behind Gas Turbine firing fuel oils. To be presented by: Pascal Fontaine, Product Manager, CMI Liege Belgium. Xavier d Hubert, Business Development Mgr East & Central Europe. CMI Groupe Avenue Greiner, SERAING (Liège) BELGIUM