Advanced technology for Nordjyllandsværket Unit 321 May 1998
In 1992, Danish utility I/S Nordjyllandsværket decided to add a supercritical, high efficiency coal fired unit at its power plant near Aalborg in Denmark. Construction is now complete, and start-up scheduled for October this year. Equipped with one of the highest efficiency steam turbine units in the world, and full flue gas cleaning, it will set new standards in power and heat supply.
The Nordjyllandsværket power station at Limfjorden near Aalborg is owned by nine partners representing the power distribution companies in the northern part of Denmark. In addition to producing power for Elsam, the new coal fired Unit 3 will supply district heating to two local district heating companies in the area.
Elsam, a partnership owned by the six Jutland-Funen utilities, supplies almost all of the heat and power consumed west of the Danish "Great Belt". Denmark's first supercritical unit at the Skærbækværket power plant in Fredericia, which started operation last year (see MPS March 1995 and June 1997), also supplies Elsam.
Nordjyllandsværket Unit 3 is the first Danish power plant that from the start of commercial operation is equipped with full flue gas cleaning, i.e. NOx, dust and SO2 removal.
With energy market liberalization demanding enhanced cost effectiveness of heat and power supply, efficiency, environmental compatibility and low maintenance costs were high priorities in determining the plant design criteria. The following concepts and details were therefore chosen for Unit 3:ultra supercritical steam data double reheat 13-stage condensate and feedwater preheating competitive district heating heat recovery from all component coolers extensive use of frequency controlled motors a high level of desulphurization industrial symbiosis use of highly corrosion resistant material in all sea water systems to minimize the surface treatment costs a high degree of automation very effective process control and monitoring systems sewage treatment for water recycling rain water collection for use in the desulphurization process use of low quality water.
The boiler for Unit 3 has been designed by FLS miljø/Burmeister & Wain Energi, and was constructed by a consortium of Aalborg Industries, Burmeister & Wain Energi (BWE) and Vølund Energy. It is an ultra supercritical once-through Benson tower type boiler with a spiral wound evaporator and double reheat. Main steam is regulated by sliding pressure operation and fully open turbine valves. The boiler has a 12.25 m square cross-section and is 70 m high. The double reheat cycle is necessary to achieve high efficiency and keep the steam humidity low at the inlet of the LP turbines, especially in combined heat and power mode.
The boiler is equipped with 16 BWE 4 attached flame (AF) burners. They are coal/oil dual fired and nominal burner load is 70 MWth.
The burners are arranged in the corners at four levels. There is an over burner air (OBA) nozzle above each one, plus an over fire air (OFA) system above the upper level of burners. The burners have four air flows enabling internal air staging at each burner as well. This concept combines the advantages of low NOx burners and in-furnace air staging. The NOx content in the raw gas at 100 per cent boiler load is 170 to 200 mg/MJ.
The OBA system and the flue gas recirculation (FGR) together control the IP outlet temperatures. Under certain circumstances the FGR can be introduced through the OFA system in order to control the flue gas temperature at the furnace outlet. The FGR feature is primarily intended for use in the oil firing mode.
The heating surfaces of the boiler are arranged to ensure efficient cooling of the flue gases. Beyond the water walls and separators, steam passes a screen and a HP pre-superheater which serve to protect the HP and IP1 final superheaters. Steam side imbalances, caused by direct thermal radiation from the combustion chamber, and the flue gas side temperature imbalances are thus moderated by the flow through the pre-superheater.
Low steam side imbalances are important in limiting high temperature corrosion of the final superheater. All final superheaters are in parallel flow and are kept small so that the steam temperature rise from the last spray type attemperator to the outlet is moderate. This arrangement limits the development of temperature imbalances between the individual heating surfaces and tubes.
The steam parameters at Skærbækværket and Nordjyllandsværket called for better alloy steels to be used compared to previous plants. Special attention was paid to the selection of materials for the final stage superheaters, evaporator, HP outlet header and the pre-separator to limit creep and high temperature corrosion. The ferritic and martensitic alloys 15Mo3, 13CrMo44, 10CrMoV9 10 and X20CrMoV12 1 are used for steam temperatures up to 540°C. 13CrMo44 was chosen for the evaporator and upper pass of the boiler because a material was needed that did not require stress relieving of the welds in the membrane walls.
Above 540°C, austenitic alloys have been used for the superheater tubes. A fine-grained version of the SA213-TP347H-type steel was chosen because of its resistance to steam side oxidation and high outside temperature corrosion. Headers and connecting pipes in the high temperature sections are made from the martensitic X10CrMoVNb9 1 (P91) steel with strength values based on ASME codes.
For the steam turbine, GEC Alsthom opted for an impulse design which allowed expansion from 285 bar (turbine inlet) to 7 bar in three single flow steam paths: VHP steam path from 285 bar to 78 bar back to the boiler; HP steam path from 74 bar to 20 bar back to the boiler; IP0 steam path from 19 bar to 7 bar further to the IP1/IP2 module.
The two HP and IP steam paths are combined in a common HP/IP module producing an efficient, compact design. The asymmetric double flow IP1/IP2 are configured to suit the district heating requirements.
VHP module: this is fed by two inlets in total injection mode and has a single exhaust. Expansion from 285 bar/580°C to 78 bar is achieved in 14 stages. For reliability, the inlet and outlet are directly welded onto the outer casing. The VHP module is composed of three casings: nozzle boxes, inner casing and outer casing.
The nozzle box is made of nine per cent Cr cast material. The design was chosen to cool down the rotor first stage and inner casing, achieved through a reverse cooling flow caused by the negative reaction design of the first stage. The nozzle box design was chosen to lower the steam conditions within the inner casing and so reduce the wall thickness and bolt size required for the inner casing.
The rotor is made of a ten per cent Cr material. It has a bearing span of 4550 mm and weighs 10 425 kg. The impulse design has allowed a moderate hub diameter to be chosen bringing good dynamic behaviour through reduced thrust on each stage. This, combined with large fillet radii and good creep fatigue characteristics ensures good operating flexibility.
In the VHP steam path, the moving blades are composed of 12 per cent Cr creep resistant material with an integral shroud. They are assembled onto the rotor with a continuous contact at the shroud to provide a damping effect and to avoid dangerous resonance.
The volumetric flow at the VHP inlet is small and so it was necessary to reduce the effect of secondary losses. As far as the moving blades are concerned, this has been achieved through a small root diameter.
Small chord guide vanes were selected for the first diaphragms. The first diaphragms are made of 12 per cent Cr creep resistant material while the last diaphragms are 12 per cent Cr material to match the thermal expansion coefficient of the rotor.
The inner casing is in two halves, bolted at its horizontal joint by Nimonic 80 A and 12 per cent Cr bolts. The material is identical to that used for the nozzle box. Perfect centering of the static part onto the rotor is ensured through support at the horizontal joint. It is protected from intense convection by the extension ring of the diaphragm and the nozzle box.
The steam conditions between the inner and outer casing were chosen to avoid strong convection on the inner casing, preventing high thermal stresses and distortions. Hot steam leakage at the inlet gland carrier is extracted outside the outer casing and recycled to the HP/IP module for cooling purposes or alternatively to the VHP exhaust.
HP/IP0 module: this comprises a nine stage HP single flow section for steam expansion from 74 bar to 20 bar, and a six stage IP single flow section for expansion from 19 bar to 7 bar. It is supplied by two inlet pipes for both the HP and IP0 steam paths, and has two casings: an inner casing and an outer casing with thermal shields at the HP and IP0 inlets. Both HP and IP0 inlets are total injection mode, with one inlet in the upper part and one inlet in the lower part.
Since the cycles are double reheat, the HP and IP0 inlet pressures are quite low compared to single reheat cycles; this implies a large volumetric flow. HP inlet flow is 12 m3s and IP inlet is 44 m3/s. The total output of the plant in full condensing mode is 411 MW but the flow at the IP0 inlet is equivalent to the flow at the IP inlet of a 850 MW supercritical single reheat unit.
IP1/IP2 module: this consists of two single-flow steam expansion sections. It has a total of six IP1 stages and eight IP2 stages. Steam enters at 7 bar and 425°C via two inlet pipes. The IP1/IP2 module is composed of one central cast casing welded to two fabricated exhaust ends.
The fabricated exhausts are connected to the district heat exchanger in the lower part and to one LP module in the upper part for each exhaust end. Because of the steam temperature at the inlet, a super clean 3.5 per cent Ni rotor is used. For optimum design in efficiency, operating flexibility and maintainability, the rotor rests on two bearings.
LP1 and LP2 modules: these are of a standard design. A cooling flow is injected in the LP steam paths when operating on the maximum district heating supply.
Steam chests: these are made of the nine per cent Cr steel material used for the inner casings. The stems are made of austenitic material and the cover bolts are of Nimonic 80A, as for the inner casings.
Each of the VHP, HP and IP steam inlets is controlled by two combined main stop/control valve assemblies. The stop and control valves are separate, though mounted in series in the same chest.
Steam turbine control system: this governs the normal speed and load/frequency control, and additional controls required by the particular operating modes of this machine have also been implemented. The machine has a programmable digital electro-hydraulic control system which uses the Microrec range of microprocessor based turbine governors.
Flue gas cleaning
The advanced flue gas cleaning system of the unit comprises three separate operations. Seen upstream of the flue gas, these are:NOx removal (high dust SCR) Dust removal by electrostatic precipitator (ESP) SO2 removal (wet scrubbing)
The plant was designed, constructed and built by FLS miljø/BWE and Deutsche Babcock Anlagen GmbH. The DeNOx catalyst, which is of high dust type, was designed and produced by Danish company Haldor Topsøe A/S.
The SCR DeNOx reactor is integrated between the boiler flue gas outlet and the air preheater. Based on a DeNOx efficiency of over 80 per cent, the NOx content in the cleaned gas is less than 40 mg/MJ.
The plant consists of the following main components: an injection system for NH3 placed directly after the boiler flue gas outlet; an SCR DeNOx reactor with catalyst and sootblowers; and an NH3 supply plant.
The DeNOx reactor comprises three catalyst levels, of which two are provided with catalyst mass from the start of operation, while the third level is intended for the installation of an extra catalyst layer, when deactivation of the first two layers demands it.
The ESP, supplied by Danish company FLS miljø a/s, is installed right after the regenerative air preheater. The flue gas passes to the ESP through two ducts which are connected to the inlet box arranged with a number of screens for equal gas distribution.
The ESP will remove more than 99.9 per cent of the incoming dust and reduce the content to less than 50 mg/Nm3 in the flue gas leaving the filter. The removed dust is stored in a flyash silo for transportation by truck to the Aalborg Portland cement factory.
A special feature in the filter is the water washing system enabling cleaning of all the internal mechanical installations before entering the filter for inspection and maintenance. The system consists of 20 short retractable lances and drives, which can be connected in turn to the pump installation, to inject about 175 m3/h through each lance. The water is collected via the dust hopper system, cleaned and reused.
The flue gas desulphurization process is a single loop in situ oxidation process developed by Mitsubishi. This was also designed, constructed and built by FLS miljø. The level of desulphurization is higher than that demanded by the Danish authorities. The flue gas is cleaned through one absorber system with the ability of using two different absorbing agents – chalk and limestone.
Nordjyllandsværket power plant is situated close to the Aalborg Portland cement factory, enabling use of the chalk present at the cement factory. An industrial symbiosis has thus been created between Nordjyllandsværket and Aalborg Portland. Chalk is transported to the FGD plant and converted into gypsum in the absorber. The gypsum is then returned to Aalborg Portland and is used in the manufacture of cement, reducing the need for imported natural gypsum.
Limestone is held in a 500 m3 silo, for use as a secondary absorbing agent if required.
In the FGD plant the water consuming processes are divided into two groups based on the water quality needed. Low quality water is obtained through the collection of rain, process and cleaning water together with the use of seven wells. Medium quality water is based on three other wells located elsewhere.
The FGD plant is a very compact design. The gas/gas heater is arranged and supported on top of the absorber, reducing the amount of duct work to a minimum. The lined outlet duct enters the stack at a level of 39 m, saving several meters of stack liner.
The inlet damper and the bypass damper, arranged with only 1 m of bypass duct, are hydraulically driven with a back up of hydraulic accumulators in case of black out and the need for flue gas bypass. The oxidation of the slurry is created by two arm rotating spargers serving also as agitators to prevent settling in the lower part.
The erection of the plant was based on the unusual engineering challenge to deliver and install the 600 t absorber in prefabricated and painted condition and with the gangways already mounted. The task was carried out successfully by FLS miljø a/s in one week, including the mounting of the gas/gas heater on top of the absorber.
The hydrogen pressure of the generator is 4.5 barg. The stator is water cooled, the rotor is hydrogen/water cooled and the exciter equipment is air/water cooled. The water cooler has been designed for a cooling water inlet temperature of 30°C and an outlet temperature of 45°C.
Generator protection is based on digital multifunction relays and for safety reasons, protection has been divided into two separate systems. The generator is fitted with an SF6 circuit breaker designed for a continuous current of 16.4 kA. This provides very rapid disconnection if any faults occur in the generator, the generator transformer or the auxiliary transformer.
The apparent power of the generator transformer is 485 MVA and the ratio is 421.3/21 kV. The auxiliary transformer is rated at 50 MVA.
As a generator circuit breaker is used, the generator transformer and the auxiliary transformer are also used as start transformer, so a fast switch over system is not needed. The apparent power of the reserve transformer is 20 MVA. The ratio is 10.5/6.4 kV. During overhauls and operation interruptions the transformer is used to supply the unit. It is not possible to start the unit via the standby transformer.
The rating of the 10 kV auxiliary power supply is 600 to 3000 A, with a shortcircuit level of 31.5 kA. Normally, the auxiliary transformer supplies the 10 kV system, which can be divided into two systems via a circuit-breaker and an interconnection. The consumers are equally distributed on the two systems so that operation with approximately 50 per cent load is possible with only one system in operation.
If it is not possible to supply the unit from the auxiliary transformer, it can be started and kept in operation with around 70 per cent load by supplying the 10 kV system directly from the 32 MVA gas turbine emergency start system at Vendsysselværket.
All motors of more than 1 MW are water cooled and the coolers have been designed for an inlet temperature of 30°C and an outlet temperature of 45°C. All motors of more than 700 kW are supplied by the 10 kV system. During construction of the plant, efforts were made to use two 50 to 60 per cent units everywhere so that reduced operation is possible even if only one unit is in operation.
The control voltage of all the power control circuits is supplied by an uninterruptible power supply (UPS), and if power fails for more than 2 s, all the controlled power supplies for motors will be switched off.
The 10/0.69 kV and 10/0.4 kV transformers are configured to standby for each other as there is a reserve connection between the related systems. All connections from the transformers to the switch gears, and mutually between these, are Cu bus bar connections. In the switch gears the main part of the system has been made in draw-out and instead of fuses, circuit breakers and auto protection have been used.
An integrated control system is provided for the remote control and monitoring of the electrical supply systems. This enables all the main switch gears of the unit, from 10 kV to 24 V DC, to be operated. Furthermore, the 6 kV busbar for the auxiliary systems of Vendsysselværket is included in the station control system. In 1999 the remote control and monitoring of the 32 MVA gas turbine system will be connected.
All events in the alarm and log of the system are also handled via the station control systems. In situations where parts of the facilities of Nordjyllandsværket need to be supplied from the gas turbine system in an island mode operation, it will be possible to synchronize the gas turbine to the network via the station control system.
The fire alarm system has been established as a colour graphics alarm and presentation system, where all the detectors are addressable. In case of fire, for example, it is possible to follow the development of the fire as analogue signals of the fire alarms can be read. The TV monitoring system consists of 60 cameras and eight monitors. Activating a fire alarm automatically switches on the TV monitoring of the area in question.
Process control system
Unit 3 is controlled and monitored by Hartmann & Braun's latest process control system "Symphony" with a system structure.
The following packages are included in the "Performer" plant management system:Process management system: ConDas Life time monitoring: ConLife Performance calculation: ConPec Accounting and logs: ConCal
The man machine interface (MMI) system – "Maestro" – consists of eight workstations configured as four redundant pairs, connected to seven operator stations in the central control room. With the MMI the power station operators have access to plant-wide information. With user tailored process displays, alarm summaries, and historical and real-time trend displays Maestro provides immediate access to process status and operations information. Multiple priority alarms allow efficient response to abnormal transient conditions. Operator configurable displays enable situation dependent groupings of critical data elements. Maestro displays provide on-line status and trouble shooting for Symphony systems.
The automation level of the "Melody" automation system comprises modular controllers, intelligent network interfaces and a range of I/O modules. It offers functions for data acquisition and signal conditioning as well as powerful open and closed loop control, sequence control and monitoring. The Melody controller uses a 32 bit processor for maximum computing power and modular scalability.
Features such as time stamping, monitoring, filtering and HART communication are implemented directly in the I/O modules. The system comprises approximately 2800 analogue and 1400 binary inputs and controls about 1150 actuators, motors and other output related consumers.
The network uses redundant, high speed serial communication with inherent capability for remote communication. Flexible communication to the I/Os is provided via the Fnet. HART communication is system integrated, including configuration and diagnostics through the network.
The engineering system for C&I, "Composer", comprises all functions necessary for a plant wide system configuration, documentation and maintenance. The system is able to define process points and loops, to design functions, to size the system and to arrange the cubicle layouts and it includes the functions for service and diagnostics. A further application within the Composer engineering system is the documentation and information management which simplifies the viewing of all data, including the integration of third party documents.
For safe and reliable operation of Nordjyllandsværket Unit 3 the central control room operators have four mouse operated operator stations coupled to ten VDUs and three operator stations coupled to large screens, as well as a conventional control desk with mimic diagrams, push buttons, analogue and digital indicators and optic alarms for the most important parts of the process, approximately ten per cent of the total number.
When Unit 3 starts commercial operation, it will supply district heating to both the Aalborg/Nørresundby area and part of the minor local towns north of the Limfjorden.
The bleed steam from the two outlets of the double flow asymmetric IP turbine supplies the two district heaters, operating at two different pressures. The outlet temperature of the district heating water is controlled by throttling the bleed steam to DH 2. At high heat load compared to the actual power load, the necessary ³p over the throttle valve is maintained by stemming up the steam pressure in the cross over piping by using the LP1 and LP2 flaps.
The building-in of a district heating accumulator tank means that the dependence between the district heating requirement and the district heating production is eliminated, which provides a number of operational advantages related to both power and CHP production.
When maximum power production is required, the CHP mode can be changed to power only mode and the district heating requirement will then be covered by the DH accumulator. Similarly, the DH accumulator can be used in the opposite situation where a low power production is required at the same time as a high district heating requirement.
It is possible to produce district heating optimally as regards operating economy by letting the heat production follow one of the broken no loss lines. By this any throttle loss over the LP1 and LP2 flaps is avoided and the lowest Cv value and also the lowest power loss in relation to the heat produced is achieved.
Component cooling: the design philosophy of the component cooling system has been to recover as much heat from the coolers as possible. The amount of heat recovered from the coolers is 10 MJ/s at 100 per cent load. To optimize the net efficiency gain, the outlet temperature of the component cooling water from each cooler has been increased as far as possible without creating additional losses in the components to be cooled. For that reason all component coolers are designed for greater temperature increases than would normally occur. In this case they are designed for inlet/outlet temperatures of 30°C/45°C.
The recovered heat is primarily utilized for heating the buildings and in the boiler house to heat combustion air and save bleed steam to the air preheaters. Secondly, the recovered heat is used for preheating the main condensate just after having passed the condensate polishing plant.
The decision to build Nordjyllandsværket was made in 1992. Nordjyllandsværket's own staff have been in charge of the project management and multi-contract procurement. They have also been responsible for mechanical and electrical design and engineering with assistance from Elsamprojekt. The design and engineering of the civil works have been performed by the Danish company Ramboll, which has also assisted with the site supervision of the civil works.
The project was implemented as multiple small contracts in accordance with the EU directives for procurement of civil works, mechanical and electrical equipment. Both Nordjyllandsværket and Elsamprojekt have long experience of project implementation according to this principle, which often results in lower investment costs than projects with a few large contractors.
The new Nordjyllandsværket Unit 3 is due to enter commercial operation on 1 October 1998. With the focus being on plant efficiency, environmental capability, performance of the chromium steels used in the plant, and the improved cost effectiveness of power and heat supply, the unit is likely to set new standards for coal fired plants operating in the deregulated market.