912 MW Supercritical Boiler for the next Coal Fired Plant Generation

Transcription

1 912 MW Supercritical Boiler for the next Coal Fired Plant Generation Konrad Ebert, Dr. Michael Kübel (1) Heinz Lorey, Wolfgang Michele (2) (1) EnBW Kraftwerke AG, Stuttgart/Germany (2) ALSTOM Boiler Deutschland GmbH, Stuttgart/Germany Abstract Coal-fired power plants continue to play an important role for the European energy supply, both in terms of securing this supply, as well as keeping the cost of electricity at an affordable level. The remarkable increase of the renewable energy sources contribution to the electricity supply over the last years has posed additional requirements in terms of flexible operation of coal-fired power plants. In order to cope with those economic, operational and environmental requirements today and in the future, advanced plant design and materials are essential. The new Karlsruhe RDK8 power plant, fired with bituminous coal, has an electrical output of 912 MW el. The overall plant efficiency is over 46 %, and the additional combined district heat production (max. 220 MW th ) leads up to 58 % of fuel utilization. The specific CO 2 -emissions are far below the European and global average. High boiler steam parameters (603 C / 285 bar, 621 C / 60 bar for the HP and RH section respectively), low emissions, a wide coal range, and the use of advanced materials are the main significant features of this steam generator. Erection started in spring The pressure tests were completed successfully in May and June Commissioning of the power plant started in This paper gives an overview of the RDK8 boiler design and the project realization with the focus on advanced boiler design. The steam turbine is also described briefly. 1

2 The RDK Power Plant The Rheinhafen Dampfkraftwerk (RDK) Power Plant is located in the City of Karlsruhe at the river Rhine (Figure 1). The history of the power plant goes back in the 1950s when the first bituminous coal-fired units started their operation. Today a capacity of 1208 MW is installed in total comprising of four power blocks: Unit 4 is a modern Combined Cycle Plant, units 5 and 6 are two oil/gas fired units, currently in cold reserve and unit 7 is a 550 MW bituminous coal fired unit. The Power Plant also supplies the city of Karlsruhe with district heating. In December 2006 EnBW decided to enlarge the site by a new 912 MW unit. The new unit, taking the name RDK8, is expected to be worldwide a milestone in terms of efficient power generation from bituminous coal, with a plant efficiency above 46 % and in terms of environmental performance with clearly lower emissions than the corresponding German and European limits. Figure 1: Overview RDK power plant (Source: EnBW) 2

3 Alstom was selected for supplying the boiler and the turbine island of RDK8 (Figure 2). This paper will mainly focus on the boiler with some reference also on the turbine. Figure 2: 3-D-Model of boiler and turbine house RDK8 Boiler design The boiler is designed for ultra-supercritical steam parameters with 292 bar and 603 C on the live steam side and 61 bar and 621 C on the reheat side at BMCR, representing the highest parameters used for such type of boiler (Figure 3). This is based on the once through technology which started commercially in the 1970s and gradually developed to higher steam parameters and to supercritical steam cycles while the main components remained but size and the pressure part material was further developed. [1] 3

4 Live Steam 312 bar (Design Pressure) 603 C 652 kg/s (2,347 t/h) Reheat Steam 77 bar (Design Pressure) 621 C 542 kg/s (1,951 t/h) Feedwater 305 C Fuel Bituminous Coal (Import) Figure 3: Side Elevation of boiler The boiler is of tower type design and the air and flue gas systems are designed as single train systems. The capacity of the boiler is optimized in order to take full advantage of the scale of economy for single flue gas train plants using the largest Regenerative Air Preheater build. The boiler is equipped with Alstom's Low NO X Tangential Firing System (LNTFS) with tilting burners. The tangential firing system is designed as a direct firing for imported coal with 4 bowl mills. The system comprises the coal bunkers, the fuel feeding system, the milling system, the burners and the corresponding air and flue gas system and is designed for a wide fuel range. Figure 4 gives a schematic overview of the coal range that includes bituminous coal with a heating value from 23.0 up to 29.3 MJ/kg. Additionally single coals outside of this coal range can also be burnt, or mixed with coals within the range shown below. This allows on the one side an operationally and economically important fuel flexibility, but requires on the other side a corresponding design of the firing system and the boiler. There are 2 bunkers for each mill so that the coal can be mixed by the feeding system from the bunkers to the mill. 4

5 Coal range Design coal 18 Ash content [%] raw Moisture content [%] raw Figure 4: Coal range diagram The grinding and drying of the coal takes place in 4 bowl mills of the type SM29/18 equipped with dynamic classifiers. Full load is achieved with all four mills in operation; there is no reserve mill foreseen. In order to achieve a good burnout of the coal within the specified range, while keeping a low primary NO X emission a fineness of approx. 10 % rest on the 0,09 mm sieve will be achieved. This value will be controlled by the speed of the dynamic classifier. The mills are arranged on one side of the boiler. Each mill has 4 dust outlets serving one level of the burner system in the four corners of the boiler (Figure 5). Each burner level has two coal burner nozzles and three air nozzles. The division of the coal dust pipe per burner takes place directly before the burner in order to maintain a good coal dust distribution. 5

6 The secondary air is supplied via separated nozzles as lower, intermediate and upper secondary air nozzle according to the following burner array: Combined upper secondary and wall offset air Upper coal and primary air Combined intermediate secondary air and oil burner for start up Lower coal and primary air Lower secondary air Figure 5: Mill arrangement This array is located at the corner cut end of the combustion chamber and each 2 burners are combined in 1 burner compartment integrated in the water walls. The whole burner array is connected to a tilting mechanism that allows the nozzles to tilt up- and downwards. Through this tilt the fireball in the furnace can be repositioned in order to automatically control reheat steam temperature in response to load changes and furnace wall blowing cycles, or to different coal qualities and the conditions of the combustion chamber, eliminating thus the need for reheat spray injection at steady state and avoiding the corresponding efficiency losses. Figure 6 gives a schematic of the tilting mechanism and an as built picture of one burner array. 6

7 Figure 6: Tilting Burner for Bituminous Coal Due to the high steam conditions, advanced materials are used which must be resistant to oxidation and corrosion in accordance with their application parameters. The materials concerned in detail are presented in Figure 7. Showing also the heating surface structure to the colours reflects the planned material selection. On the right-hand side of the picture the header materials are listed. The economizer is completely made of 16Mo3. Reheater 1 consists of the materials 16Mo3, 13CrMo4-5, 10CrM09-10, X10CrMoVNb9-1 and S304H. For the Superheater 2 supporting tube screen the material VM12 is utilized, which is a newly developed 12 % Cr-material with a creep rupture strength similar to T91. The last 2 Superheaters and Reheater 2 are made of austenitic alloys. For reasons of corrosion protection, the austenite material has a mean chromium content of at least 18 %; in the hot sections of the Reheater 2 of at least 25 %. Furthermore, wherever the 18 % Cr-material is used the shot peened variant with a higher steam oxidation resistance was selected. Due to the high steam temperatures and the high axial loads, the material 7CrMoVTiB (T24) is employed as a wall material at the end of the evaporator spiral and in the vertical tubing part of the membrane wall (Figure 8). The supporting tubes are also made of this material for the same reason. 7

8 Separator and vessel are made of P92. For the superheaters and the reheaters, from the reheater 1 outlet onwards, also the martensitic 9 % chromium steel P92 is used as header materials. Heating Surfaces 16Mo3 13CrMo4-5 7CrMoVTiB CrMo9-10 X10CrMoVNb9-1 VM12 Super 304 H Shot blasted HR3C Eco RH 1 SH 3 RH 2 SH 4 SH 2 Header 15NiCuMoNb (Inlet/Outlet) 16Mo3 (Inlet) X10CrWMoVNb9-2 (Outlet) X10CrMoVNb9-1 (Inlet) X10CrWMoVNb9-2 (Outlet) X10CrWMoVNb9-2 (Outlet) X10CrWMoVNb9-2 (Inlet) X10CrWMoVNb9-2 (Outlet) X10CrWMoVNb9-2 (Inlet) X10CrMoVNb9-1 (Outlet) Figure 7: Material Concept Heating Surfaces T24 vertical walls (SH 1) T24 spiral walls 13CrMo4-5 spiral walls Figure 8: Material selection of waterwalls 8

9 Turbine Design The steam turbine generator used for the Karlsruhe RDK8 power plant is designed to achieve highest efficiency levels. The steam parameter at turbine inlet are 275 bar / 600 C on SH side and 58 bar / 620 C on RH side at TMCR. The five casing turbine consists of separate HP and IP turbine modules and three double flow LP turbine modules. These LP turbines are equipped with steel made last stage blades of 45 inch, taking maximum advantage of the low condenser pressure. In addition to the nine stage regenerative feed water heating system, steam can also be extracted from the LP turbines to feed the district heating system of the local municipal utility. The turbine is connected to a hydrogen/water cooled generator of 1,175 MVA (Figure 9). Figure 9: Turbine Island RDK8 9

10 Time schedule First activities on site started in March 2008 with excavation works followed by piling and civil work. The erection of the boiler started in March 2009 with the erection of the main steel structure. In January 2010 the generator stator was delivered to site. Subsequently the erection of the turbine started. After finalizing the erection of the pressure part, the pressure test was done successfully in May and June 2012 for the respective part. The next major step was the acid cleaning of the pipes and tubes in February First Fire is scheduled in April 2013 and start of trial run in January Experience from execution The permitting process started in April 2007 and in February 2008 the permitting authority, Regierungspräsidium Karlsruhe, has given approval for the start of first civil work, which could already start ahead of the other activities. Beside the careful preparation of the permitting documents, an extensive exchange of information with the public was a key for successfully passing the process. During this permitting process EnBW committed to lower emission levels for the new unit compared to the limits posed by legislation. Concerning NO X the emission level will be 100 mg/nm³ at boiler outlet and will be achieved by the combination of the low NO X firing system with a 2 layers SCR. The execution phase of this project fell within the construction peak of many new power plants in Europe. As a consequence the manufacturing of steel structure and pressure parts had to be split to different workshops, partly to workshops others than Alstom s. The coordination and control of time and quality was a demanding task. Furthermore the advanced materials used require a high degree of quality assurance, in order to avoid failures during commissioning, or service life of the unit. Especially the fulfillment of the specified welding quality requirements for the T24 material in the water walls proved to be very demanding. Extensive quality assurance effort in the workshops and on site from Alstom, EnBW and the involved Notified Body was done. In the course of this process parts of the waterwalls had to be reworked and partially waterwalls had to be produced again. The steel structure design, supply and manufacturing was split between European, Canadian and Asian sub-suppliers, while the design was performed according to EN-standards. This 10

11 split led to extensive interface management and required again a huge effort on co-ordination and monitoring in order to remain within time and fulfill the quality requirements. The main steel structure was manufactured in Europe and a new design was introduced: the main columns of the main steel structure are made in compound design with steel plates filled with concrete. An extensive program of testing had to be done to define the concrete mixture for filling up the main columns. Also the special way how to fill the sections of the column had to be worked out. Yet, the expected cost savings from such design did not materialize. Erection of the boiler house steel structure, the air and flue gas ducts, the main components like fans, RAPH, DeNO x casing and the vertical water walls was made with 3 tower cranes (Figure 10), one located at 47 m elevation on top of the bunker building which is located alongside the boiler building and designed as self standing structure. This crane set up proved to be very effective. Figure 10: Erection of steel structure and buckstays 11

12 At peak work load during pressure part erection up to 700 people worked in the boiler house and up to 9,000 welds were done per month in this period. A positive EHS result could be achieved in 2011 and 2012; there was no Lost Time Accident reported. The pressure tests of the high pressure and reheat system were done successfully in 2 steps. The testing pressure on the live steam side was defined according to the EN-standard and was 574 bar; the highest ever for an Alstom boiler. Chemical cleaning was performed for the entire water steam cycle except the spiral and vertical water walls of the boiler. These pressure parts were cleaned with high speed water flushing. In order to mitigate any potential risk of hydrogen induced stress corrosion on the T24 part of the boiler, a special procedure was followed during commissioning and start-up of the boiler, which was derived from the successful experience of commissioning the Neurath F and G units two years ago. During execution phase a further investigation was done by EnBW and Alstom in regard to the coal range and it was agreed that the coal range was enlarged compared to the original range as shown on page 5. Outlook The changing operational regime for bituminous coal fired units in Germany due to the increased supply of renewables and the need for more flexible operation of the conventional power plants [2] led EnBW and Alstom to work out further improvements to increase flexibility of RDK8. Those measures were decided during the execution phase and consisted mainly in: the reduction of the minimum once through load and the introduction of the one mill operation as a normal operation mode for the minimum load, which enables a minimum load of approx. 20 % of the nominal load Furthermore a concept on reducing the time for load changes is under investigation and is expected to be implemented also. Concerning the further schedule first firing of the boiler is planned for April 2013, followed by the first synchronization in July. After a further optimization period of the whole plant during commissioning, boiler and turbine will carry out their trial run operation in January

13 Figure 11: View of boiler house References [1] Stamatelopoulos, G.-N.; Sadlon, E.: Advancement in coal-fired power plants: Higher Efficiency using advanced materials, Power-Gen Europe 2005, June 2005, Milan, Italy. [2] Heinzel, T.; Meiser, A.; Stamatelopoulos, G.-N.; Buck, P.: Einführung Einmühlenbetrieb im Kraftwerk Bexbach und im Kraftwerk Heilbronn Block 7, VGB Konferenz Brennstoff und Feuerungen, Mai 2012, Kassel. 13