Porvoo proves 6FA niche

In early 1996, Neste Oy took the decision to construct a gas turbine power plant at its Porvoo refinery in Finland. Completed in record time, the plant became operational in October 1997 and was officially inaugurated in February 1998.

Located approximately 40 km east of Helsinki in southern Finland, the Porvoo plant supplies power and heat to meet the needs of what is the only large integrated oil refinery and petrochemical site in the country. The plant is equipped with a gas turbine and heat recovery steam generator (HRSG), and is integrated into an existing power plant at the site to form a combined cycle cogeneration plant.

The new unit is equipped with a GE MS6001FA gas turbine. Featuring advanced turbine technology, this will be only the second 6FA to have entered commercial operation in Europe.

The Porvoo power plant produces 110 t/h of high pressure steam and 72 MW of power. In total, the project cost around FIM 250 million ($45 million) and involved 100 man-hour years of employment.

The new Porvoo plant has been in base load operation since February 1998, and for Neste Oy represents a cost-effective solution to the problem of rising energy needs at its refinery.

Self sufficiency

The commercial success of an oil refinery relies heavily on its operational reliability and a high rate of capacity utilisation. Self sufficiency in power production is therefore important to competitiveness, and this factor, combined with a rising demand for steam and electricity at the Porvoo refinery during the 1990s, encouraged Neste to examine its future energy supply options.

Neste therefore commissioned engineers Ekono Energy to study the possibilities. Ekono Energy had also been responsible for designing and implementing the existing power production facilities at the site: first in 1971 with the installation of a steam power plant, consisting of two boilers and two steam turbines, and later in 1987 with the addition of a natural gas fired gas turbine plant based on a GE Frame 6B.

For the new unit, Neste and Ekono Energy originally considered the possibility of constructing an integrated gasification combined cycle (IGCC) plant based around GE Frame 9E machines and using refinery oil residue. But by the end of 1995 this concept was found to be unfeasible and Neste decided to limit the new plant to more conventional technology. Having experienced success with the gas turbine already installed at the plant, Neste commissioned Ekono Energy to design a new gas turbine power plant for the site.

In making the decision to construct a new gas fired plant, Neste decided to limit its capacity to the electricity needs of the Porvoo site, i.e. 70 MW. This decision was largely governed by two main factors. The implementation of new emission regulations in Finland dictated that the two existing boilers at the site would require refurbishment, including the fitting of low NO_x burners, in order to continue operation. A supply of steam was therefore required to replace the output of these during refurbishment.

At the time, Neste was a net importer of power from the grid, although its power purchase agreement was due to expire in late 1997. Replacing this purchase with its own electricity production could bring economic benefits as well as the ability to replace the steam output of one of the old boilers, given a suitable gas turbine. This would also allow Neste to be self sufficient in energy, thus helping to ensure the future competitiveness of its refinery facilities.

Considering these factors, the Frame 6FA gas turbine was an obvious choice for the project given its suitable power capacity and an exhaust temperature high enough to produce the same quality of 110 bar steam as the two fired boilers.

Advanced technology

The 6FA is a 70 MW class gas turbine with nearly double the output of the 6B and over half the output of the 9E. Incorporating advanced aircraft engine technology, its compact, heavy duty design addresses the need in the market for a high efficiency, medium-sized gas turbine for cogeneration and combined cycle applications. Being highly packaged, it also allows easy and quick customerization, a key factor in reducing cycle times and costs.

European Gas Turbines (EGT), supplier of the GE gas turbine to Porvoo, places considerable emphasis on customerization to reduce costs, in particular through the standardization of gas turbine accessories such as the starting system and other modules. The 6FA is the latest addition to GE's family of advanced F technology machines, and also the newest addition to the product range of EGT, GE's licensee.

The 6FA is a scaled-down version of the 7FA and 9FA gas turbine models. GE has employed aerodynamic scaling in its development of gas turbines for over 30 years. In the design of the 6FA, experience gained from GE's F technology fleet has been applied, totalling around 750 000 fired hours of operation.

The 6FA therefore benefits from a combination of proven technology and advanced aircraft engine technology.

With a simple cycle design rating of 70.14 MW and a simple cycle efficiency of 34 per cent, this high speed gas turbine is fuel flexible. Fuel switching can take place on-line with no performance loss. Net efficiency in combined cycle applications is 53 per cent. GE has estimated that the long-term reliability of the 6FA will approach 99 per cent.

The 6FA consists of 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 is close in design to that of the 7FA, with similar air extraction locations and similar alloys are also used. Compressor extraction air provides cooling for the nozzles and buckets. The can-annular combustor is the same size and configuration to that of the 9FA, although the 6FA has six combustion chambers as opposed to the 18 in the 9FA.

The combustion liner of the 6FA has a plasma-sprayed thermal barrier coating applied to its surface to improve strength and reduce metal temperatures and thermal gradients. The turbine section has internal cooling circuits to provide air cooling to all three nozzle stages and to the first two bucket stages.

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.

A number of other features are also incorporated into the 6FA to improve performance, including static honeycomb seals and coated rotating cutter teeth to provide tighter clearances. Reduced use of cooling air in the hot section of the turbine also improves life and performance.

Plant design

The first gas turbine plant at the site is based around a single GE Frame 6B gas turbine and HRSG. The availability of natural gas supplies from Russia was a major factor in the decision to install this unit. This project was completed in 1989, with Ekono Energy responsible for conceptual design, basic and detailed engineering, plant layout design, project supervision and performance tests.

The original steam power plant at the site consists of two heavy fuel oil fired boilers each producing 115 kg/s of steam at 100 bar and 520°C, and two steam turbines, one back pressure rated at 47 MWe, the other a 31 MWe condensing turbine. For this project, Ekono Energy was responsible for the complete engineering.

For the new power plant, Ekono Energy undertook the procurement planning of the gas turbine and HRSG, and in the implementation phase was the principal designer and carried out basic implementation engineering, including process, electrical, automation, layout and piping design as well as structural, HVAC and building electrification engineering. Ekono Energy also played a major part in equipment purchases.

The main fuel used at Porvoo is natural gas, with light fuel oil as a back up. The 6FA is equipped with a dry low NO_x (DLN) combustion system to ensure that NO_x emissions will not exceed 60 mg/MJ when firing natural gas and 150 mg/MJ when burning light distillate. In addition, water injection is used when firing with light distillate to maintain low NO_x levels.

At Porvoo the gas turbine is rated at 72 MW with an efficiency of 34.4 per cent at 5°C, an exhaust temperature of 576°C and a mass flow of 212 kg/s. This is sufficient to produce 110 t/h of steam at 105 bar and 540°C from an unfired heat recovery boiler.

The HRSG was supplied by Finland's Foster Wheeler Energia Oy. It is a horizontal natural circulation boiler equipped with supplementary firing. To achieve a higher steam production efficiency and to maximise steam production capacity, Neste Oy selected a dual pressure HRSG. The evaporation capacity of the HRSG without supplementary firing is 30.1 kg/s (high pressure) and 5.6 kg/s (low pressure). There is no by-pass stack.

Refinery gas, collected from the refining processes, is used as the fuel in supplementary firing. Normally, steam production without the supplementary firing is adequate for the site's needs. Supplementary firing is used as a back up when the existing boilers are not operating. Maximum evaporation with supplementary firing is 50.6 kg/s and 3.8 kg/s.

The pressure stages were determined by the existing power plant as the steam is mixed with that coming from the existing boilers, and is delivered to the existing steam turbine as well as to the process system. High pressure steam is fed into a new 105 barg steam header, which is connected to the existing steam system.

Few auxiliary systems were required for the new plant as many of the existing systems were already large enough to accommodate the new unit and only had to be extended. The plant is connected into the existing local 110 kV grid via a 90 MVA transformer supplied by ABB. The gas turbine is equipped with a GE Mark V Speedtronic control system which is integrated with the automation scheme through a display system in the local control room. The Mark V system has built-in diagnostics for trouble-shooting purposes, for example control panel and sensor fault identification.

Renovation

With the new power plant in operation, it will be possible to renovate the boilers and automation of the old plant. The two boilers which were commissioned in 1970 each produce 200 t/h of steam. They were originally heavy fuel oil fired, but have since been modified to burn natural gas.

All the burners in the boilers will be replaced, changes will be made to the superheaters and equipment for recycling the combustion gases will be installed. The automation of the whole plant will also be replaced with a Valmet Damatic control system for the boilers and steam turbines, and this will be extended to the HRSG of the new unit. There are no plans to renovate the steam turbines. The renovation project will cost an estimated FIM70 million ($12.6 million) and will be completed this year.

Commissioning

From project implementation to the handing over of the plant to Neste took almost exactly two years. The plant was completed in the autumn of 1997 and performance testing was completed successfully in December 1997. The plant has been in baseload operation since the normal burner inspection was completed in February 1998.
Tables

Table 1. GE Frame 6FA ratings, ISO, Base Load, 60 Hz