Frame 6FA cogeneration commissioned at Kirkniemi22 February 1998
In 1995, Metsä-Serla was faced with the problem of securing reliable energy supplies for its expanding Kirkniemi paper mill. The task of finding a solution was outsourced to IVO, and late last year a new combined cycle cogeneration power plant was commissioned at the Kirkniemi site. Generating 105 MWe and 120 MWth, the new plant provides almost all of the mill's energy needs.
In March 1996, IVO Power Engineering Ltd. began constructing a gas turbine combined cycle (GTCC) cogeneration plant on the site of the Kirkniemi paper mill in Finland. The plant, owned by Finnish power company Imatran Voima Oy (IVO Group), is an essential part of a wider project to expand the paper mill's production capacity. It is now fully operational and meets 80 per cent of the mill's power requirements and all of its process steam needs.
The new GTCC plant has an electrical capacity of 105 MW and a thermal output of 120 MW. In addition to supplying its output to the Kirkniemi paper mill, owned by Finnish paper group Metsä-Serla, it supplies district heating to the nearby municipality.
Metsä-Serla took the decision in 1995 to invest in a new paper machine at the Kirkniemi mill to meet rising demand for its products. An important factor in this project was the ability to secure a reliable and competitively-priced energy source for the expanded mill. Keen to focus on its core business, Metsä-Serla outsourced to IVO the task of constructing a new power plant.
The power plant is fired with natural gas, and is the largest industrial combined cycle cogeneration power plant in Finland today. It is also the site of the first GE Frame 6FA gas turbine to enter operation in Europe.
Metsä-Serla is the third largest forest industry group in Europe. It was founded in 1986 through the merger of G. A. Serlachius Oy and Metsäliiton Teollisuus Oy, two longstanding Finnish forest industry companies. The company employs around 12 000 people. Turnover in 1996 was FIM 16 billion ($3.7 billion), and the company focused strongly on its core activities in 1997.
Metsä-Serla obtains its energy from both in-house power plants and purchased supplies. Overall, wood waste and process materials supply around 52 per cent of the company's primary energy needs while fossil fuels account for 31 per cent; nuclear power 14 per cent and hydropower three per cent. According to Metsä-Serla, the new GTCC plant at Kirkniemi will reduce coal use to almost zero while the use of natural gas will rise to over 50 per cent.
The Kirkniemi mill is located in the town of Lohja, approximately 65 km west of Helsinki. Around 850 personnel are employed at the mill which produces a range of high quality paper and paper boards for magazines, catalogues and brochures. In 1995, Metsä-Serla decided to extend the production capacity of the mill by adding a new fine paper machine. This decision made it necessary to increase energy supplies to the mill, and Metsä-Serla began to examine its options.
The mill is already equipped with an existing power plant which, prior to the addition of the new paper machine, provided one fifth of the mill's energy needs. The first section of this plant was completed in 1965 and consists of an oil fired boiler and steam turbine. In 1971 the plant was extended with the addition of another oil fired boiler and steam turbine. This second boiler was converted in 1985 to a circulating fluidized bed boiler burning wood waste from the mill.
In planning new energy supplies, Metsä-Serla contemplated building, owning and operating its own new power plant. But after careful consideration, it decided that the best option would be to concentrate on its core business of paper production rather than on the development of further self-generation.
Metsä-Serla therefore entered into an energy agreement with IVO Group under which IVO bought the existing power plant and became responsible for securing new energy supplies for the mill.
IVO undertook the project in two stages. The first consisted of the construction of an oil fired low-pressure steam boiler which would guarantee the supply of steam to the paper mill when the new paper machine was brought on-line in August 1996. This boiler was commissioned in the summer of 1996 and was initially fired with heavy fuel oil until natural gas supplies to the site were secured just over one year later. This boiler was supplied by Steamservice Oy of Finland, and since the commissioning of the GTCC plant, it has become a standby unit. In addition, the oldest boiler and steam turbine have been removed from the site and placed into long-term storage.
The second stage of the project was the construction of the new GTCC plant. It was built under a $72 million turnkey contract covering the design, procurement, construction and commissioning of the facility and its connection to the existing plant.
The main engineering, procurement and construction (EPC) contract for both stages of the project was carried out by IVO Power Engineering Ltd., a subsidiary of IVO Group. Ekono Energy Ltd. undertook the conceptual design, engineering and procurement planning of the main equipment and processes. Ekono was also responsible for the overall implementation of a 110 kV substation. The main equipment suppliers were General Electric (GE), Foster Wheeler Energia Oy, GHH Borsig Turbomaschinen GmbH and Honeywell Oy. The orders for the main components were awarded in 1995. In addition, around 20 civil construction contracts were placed and the project involved over 60 subcontractors from all over Finland.
According to the energy agreement, IVO owns the new GTCC power plant, and Metsä-Serla's personnel is responsible for its operation and maintenance. Power and steam generated at Kirkniemi is sold to the mill under long-term contracts between IVO and Metsä-Serla.
Natural gas is supplied to Kirkniemi under a long-term fuel supply agreement between Metsä-Serla and Gasum Oy. The annual consumption of natural gas by the plant will be approximately 250 million m3. The back-up fuel for the plant is light fuel oil, of which 3000 m3 can be stored on-site.
The preliminary design of the power plant began at the same time as the decision on the new paper machine was made. Construction work began on 1 March 1996 and delivery of the main units took place in late 1996 and early 1997. First synchronization of the gas turbine was carried out on 9 June 1997 and of the steam turbine on 18 June 1997. Full commercial operation began on 16 November 1997.
In 1994, Ekono Energy Ltd. was commissioned by Metsä-Serla to carry out the feasibility studies of energy supply alternatives for the Kirkniemi mill. This later continued with Ekono carrying out the conceptual design, engineering and procurement planning of the main equipment for the combined cycle plant.
At the Kirkniemi site, restrictions were placed on the layout alternatives for the new plant by the presence of the factory railway and factory road. Space optimization was therefore essential to accommodate the layout of the gas turbine hall, horizontal boiler, steam turbine and associated facilities.
The power plant buildings have mostly steel or precast concrete element frames. The total volume of the combined cycle power plant building is some 70 000 m3, with a total floor area of 5900 m3.
The combined cycle island consists of a GE Frame 6FA gas turbine, a three pressure level natural circulating HRSG and an extraction back-pressure steam turbine with a condensing tail.
Natural gas is supplied to the plant via a purpose-built pipeline from Kirkkonummi 30 km away. The pressure of the natural gas at the supply interface is approximately 19 barg, and the pressure is increased to the required 26 barg at the gas compressor station located 150 m away from the power plant. The gas compressor station is equipped with two reciprocating-type compressors which were supplied by Nuovo Pignone of Italy.
The General Electric MS6001FA gas turbine at the heart of the Kirkniemi plant is the third unit of this type commissioned worldwide, and the first to be commissioned in Europe. The 6FA is the latest addition to GE's family of advanced F technology machines, and is characterized by an output in the 70 MW range, a robust design, a high exhaust energy and full packaging. This combustion turbine, a scaled-down version of the 9FA and 7FA, is available for both 50 Hz and 60 Hz operation and is aimed at a variety of mid-sized applications including combined cycle cogeneration plants such as Kirkniemi. Designed to operate at a firing temperature of 1288°C, the Kirkniemi gas turbine installation will deliver 72.6 MW at an ambient temperature of 5°C.
The 6FA is a 0.69 aerodynamic scale of the 7FA and has a simple cycle design rating of 70.14 MW. It is a high speed gas turbine designed to perform with a simple cycle efficiency of more than 34 per cent and a 53 per cent net efficiency in combined cycle applications. As with other F technology units, the 6FA is highly fuel flexible, and at Kirkniemi will burn natural gas with light fuel oil as a back-up fuel. Fuel switching can take place on-line with no performance loss. In addition, 6FA units are equipped with a triple-redundant microprocessor-based control system to ensure high reliability. GE estimates the long-term reliability of the 6FA to approach 99 per cent.
GE has used aerodynamic scaling in gas turbine development for over 30 years. By scaling proven technology and combining it with advanced aircraft engine technology, the 6FA benefits from experience gained in more than one million fired hours of operation.
The 6FA includes an 18-stage compressor, six combustion chambers and a three-stage turbine. The shaft is supported on two bearings and five casings form the structural shell. The axial compressor uses alloys similar to those used on the 7FA in its rotor construction, and the compressor air extraction locations are similar to those of the 7FA. Compressor extraction air, which does not require external coolers, provides the cooling for the buckets and nozzles. The cooling circuit for the buckets is internal to the rotor and there is no loss of air in its transfer at stationary to rotating seals.
The 6FA has a can-annular combustor of the same size and configuration as the 9FA. However, the 6FA is equipped with six combustion chambers as opposed to the 9FA's 18. The 6FA and 9FA share common head end components such as nozzles and swirlers, and the 6FA's combustion liner cap includes multiple fuel nozzles to reduce combustion wear and inspection intervals. Kirkniemi's 6FA is fitted with a dry low NOx (DLN) combustion system to ensure that NOx emissions will not exceed 60 mg/MJ when firing natural gas. Emission levels of CO are 15 ppm with the DLN system. When burning light fuel oil, water injection will be used in order to cut emissions.
The combustion liner of the 6FA is modified to accommodate minor differences in cross fire tube arrangement and transition piece interface. A plasma-sprayed thermal barrier coating is applied to the liner's surface to improve strength and reduce metal temperatures and thermal gradients.
The three-stage turbine section has internal cooling circuits to provide air cooling to all three nozzle stages and to the first two bucket stages. All the turbine buckets are composed of GE's GTD-111 alloy which provides superior rupture strength, low cycle fatigue strength and corrosion resistance. The first and second stages are vacuum plasma spray coated with GE's GT-33 coating which provides hot corrosion resistance. The third stage bucket uses a diffused bromide coating for protection against low temperature corrosion and oxidation.
The first stage nozzles are investment cast from GE's cobalt-based alloy, FSX-414 for improved oxidation resistance and weldability. Second and third stage nozzles are cast from GE's GTD-222 alloy for high creep strength.
The 6FA also incorporates a number of other features which enhances performance. Static honeycomb seals and coated rotating cutter teeth provide tighter clearances in locations that significantly affect performance, and tighter compressor blade and bucket tip clearances are also maintained by equivalent thermal masses distributed around the periphery of the casings, which provide compensation for the cold flanges at the split lines. This produces tighter tip clearances during operation. Reduced use of cooling air in the hot section of the turbine also improves life and performance.
The turbine is connected to an air-cooled generator via a gearbox. Class F insulation on both rotor and stator provides dielectric and mechanical strength. The fast acting static excitation system provides high initial response, which is needed under fault conditions.
Supplied by Foster Wheeler Energia Oy, the heat recovery steam generator (HRSG) is a three pressure evaporation unit with an additional district heating economizer. It is designed as a horizontal unit with the ability for supplementary firing to increase the amount of high pressure steam produced. It uses natural gas as the primary fuel with light distillate as the back-up fuel.
In the case of gas turbine overhaul or failure, the diverter damper between the gas turbine and the HRSG is closed and the unit operates on 100 per cent boiler output using forced-draught air fans and gas burners in the same way as a conventional boiler. Correspondingly, during periods of boiler outage, the diverter damper can be closed to allow the gas turbine to operate in simple cycle with exhaust gases passing directly to the by-pass stack.
During normal operation, hot exhaust gases from the gas turbine at 597°C are fed to the HRSG, which can produce a maximum of 50 kg/s of high pressure steam at 530°C and 81 bar. Without additional burning and with the gas turbine at baseload, the evaporation is 30 kg/s. Intermediate pressure steam at 11 bar and 250°C is produced at around 3.3 kg/s, and 1.2 kg/s of low pressure steam is produced at 3 bar and 160°C. When operating as a stand-alone boiler unit with the auxiliary fans, the amount of steam obtained is 45 kg/s.
The HRSG can also produce 14 MWth for district heating and output on supplementary firing is 54 MWth.
According to Foster Wheeler, high usability was the primary concern in designing this unit. Previous experience with a similar project played a large part in the design of the Kirkniemi unit, particularly of the fresh-air burning device. During changeover from gas turbine to fresh-air burning, the activity of the paper mill is not disturbed.
The high temperature of the gas turbine exhaust gases set special requirements for the selection of materials for the HRSG, but also made it possible for an exceptional layout of the heat surfaces. Part of the high pressure evaporator is built as a radiation shield before the superheater surfaces. Because of the evaporator surface, the changes in the amount of steam and the temperature are smaller during firing mode changeover because the burner output can be increased faster than it could have been without the radiation shield.
The changeover time from gas turbine use and nominal output of auxiliary burning to a corresponding output level with fresh air burning was specified as eight minutes. In practice, a changeover time of less than seven minutes has been achieved at Kirkniemi. In addition, the procedure takes place in such a manner that the operating temperature of the gas turbine does not fall below the minimum permissible temperature of 480°C during the changeover.
This short changeover time also required the special design of the burning equipment components. The installation includes two fresh air blowers instead of one larger one, synchronization of the fresh air valves with the by-pass valve through a common hydraulic aggregate, exceptionally large cylinders, and the use of a steam trap for preheating the fresh air.
The burners were supplied by Dutch company Rodenhuis & Verloop b.v. The Kirkniemi burner is the largest bi-fuel burner supplied by Rodenhuis, and there are a total of five burners in the gas flue with a nominal output of 175 MW.
The by-pass pipe and fresh air channel valves were supplied by Hermann Rappold & Co. GmbH of Germany. At the bottom of the by-pass valve there is a diverter valve. The flap door, hinged at the top, closes the channel to either the by-pass pipe or the boiler. The driving power of the valves is derived from the hydraulic aggregate which makes a swift reaction time possible when the boiler safety guard is triggered. In addition to the hydraulic pumps, the operation of the hydraulic aggregate has been ensured with hydraulic accumulators.
The design temperature of the diverter valve is exceptionally high at 650°C in continuous operation.
When the boiler was undergoing initial start-up, a planned programme of functional tests was performed. As a consequence, some changes were made to the process guidance in order to improve its functionality. Furthermore, the coupling between the furnace of the boiler and the actual boiler part was improved so that it would better endure the forces created during the changeover in the operating method. On the basis of the test run experience, a special flow equalization area was modified in front of the radiation shield, after which the temperature distribution in the superheater packages levelled off optimally.
The steam turbine generator set was designed and manufactured by GHH Borsig Turbomaschinen GmbH. It has an average electrical output of 24 MW and a maximum output of 38 MW. GHH Borsig's contract also covered the installation of the unit complete with auxiliary systems and balance of turbine plant equipment.
The steam turbine is a single casing extraction back pressure type turbine with a condensing tail. It is a single-flow reaction turbine with one internally controlled 3 bar back pressure extraction at 44 kg/s and 160°C, and a three point sliding 11 bar bleed at 7 kg/s and 250°C. Live steam flow at the turbine is approximately 45 kg/s at 80 bar and 525°C, and the exhaust flow of the condensing tail is 15 kg/s.
The turbine drives a 4-pole synchronous generator via a gearbox. The generator was manufactured by ABB Industrial Systems AB. The surface condensing plant is designed for a cooling water inlet temperature of 3°C. The turbine generator set has been designed as a packaged unit and is equipped with oil from a combined lube, control and jacking oil unit.
The unit is also equipped with GHH Borsig's Turbolog DSP digital control, protection and monitoring system to meet flexibility, speed, accuracy, reliability and availability requirements.
The power plant consumes approximately 0.7 m3/s of cooling water, most of which goes to the direct cooling water circulation of the steam turbine condenser. The remainder is used to cool the closed cooling water system which comprises the air condensers of the plant generators and the lubricating oil systems of the turbines.
The paper mill's raw water requirement is around 10 m3 per tonne of paper, which is roughly the same as the power plant's cooling water volume. This has made it possible to combine the power plant's cooling water system and the mill's raw water intake. Cooling water is taken from Lake Lohjanjärvi and is first used as the power plant's cooling water and then is pumped to the paper mill's raw water storage tank. Metsä-Serla attempts to minimize water consumption at the mill and each litre of water circulates 18 times before returning to the lake.
Operation and control of the plant is carried out by a Honeywell DCS system supplied by Honeywell Oy of Finland. The system is an Alcont 3000x automation system which includes a process computer system for monitoring the process and collecting historical operations data. Alcont 3000x is a new generation open distributed control system which extends real time control to the entire plant by unifying process, production and business management.
The system monitors the entire combined cycle plant as well as the CFB boiler and the standby boiler and steam turbine in the old power plant. All these systems can be viewed in the main control room on two large overhead screens.
The Kirkniemi mill site is connected to the Finnish grid via the Virkkala substation with two 110 kV lines. The combined cycle power plant supplies around 80 per cent of the mill's electricity requirements and the remaining demand is taken from the grid.
The main transformers of the combined cycle plant and the mill are connected to 110 kV gas insulated switchgear (GIS) where 'inner' transmission between the mill and the power plant.
The generator of the gas turbine, with a rated output of 90.7 MVA, is connected to to the power plant's main transformer by means of isolated busducts and the generator breaker. The generator of the steam turbine has a rated output of 44.2 MVA and supplied its power directly to the mill's 10 kV distribution system.
The plant service system of the combined cycle power plant is supplied from busbars of the gas turbine generator, the plant service voltages being 3 kV and 400 V. The major single loads are the starting motor and feed water pumps of the gas turbine.
In order to maximize plant efficiency, the 6FA is operated at full capacity (i.e. baseload). During normal plant operation, the existing CFB boiler operates at constant baseload, supplementing steam production. This feeds steam into the same high pressure network as the HRSG. High pressure steam is fed into the steam turbine in the combined cycle island. The CFB boiler is fired with bark, waste fibres, coal and oil, and generates 25 kg/s of live steam at 80 bar and 510°C.
The high pressure and low pressure steam systems of the combined cycle plant and the existing power plant are connected. One steam turbine from the existing plant and the low pressure steam boiler act as standby units for use if the steam turbine or HRSG in the new plant fail.
Low pressure steam from the combined cycle island is fed into the steam network of the paper mill at a pressure of 11 bar. However, the requirement of low pressure steam can vary significantly, particularly in the event of failures and start-ups at the mill. This need is accounted for in the design of the steam network, and the power plant is fitted with a 300 m3 capacity steam accumulator in addition to the condensing part of the steam turbine and the auxiliary accumulator located in the old power plant.
The low pressure steam boiler which was constructed as the first phase of the Kirkniemi power project now acts as the standby boiler. It is located in a warm hall and so can be started quickly in the event of steam production failures. This has a thermal capacity of 54 MW and burns natural gas and heavy fuel oil. It generates 21.4 kg/s of live steam at 13 bar and 235°C.
In the event of a steam turbine failure, high pressure steam is let down by means of pressure reduction stations so that it can be fed into the steam network of the paper mill.
An outsourcing solution
The Kirkniemi GTCC cogeneration plant began full commercial operation on 16 November last year when it was officially handed over to the owner. Performance testing of the plant was due to take place during the last two weeks of January after which the final test report would be available. The tests cover the main guaranteed parameters of the plant including output, efficiency, noise and vibration, and environmental performance.
With the new plant commissioned, the Kirkniemi paper mill now has a reliable and competitively-priced source of energy which accommodates most of its needs. In addition, Metsä-Serla has been able to centralize and rationalize the maintenance of the plant by using the maintenance resources of the paper mill, and at the same time can focus on its core business of paper production. With such obvious benefits, there seems little doubt that outsourcing was the right solution for Metsä-Serla.
TablesTable 1. Main parameters of the 6FA gas turbine in simple cycle Table 2. Project schedule Table 3. Main technical parameters