Extended baseload is no Greek mythology

A 390 MW combined-cycle power plant at Thessaloniki in northern Greece has more than twelve months of successful operation behind it. In that time it has been able to take advantage of low hydropower potential after a dry autumn and a good emissions performance to extend baseload operation opportunities.

Eric Jeffs

Thessaloniki, situated 500 km north of Athens, is a major centre of the Gree oil industry. It is home to an Hellenic Petroleum refinery and a number of petrochemical plants. In late 2005, a 390 M combined-cycle power plant, which represents the country’s first independent power station (IPP), came online to supply electricity to the Hellenic Petroleum site.

The IPP is owned by Thessaloniki Power, a company created by Hellenic Petroleum. The plant also includes a 400 kV substation, from which the output is sent to HTSO, a power transmission company that is the high voltage grid business of the former Public Power Corporation.

The power station is otherwise completely independent of the refinery, which has maximum power demand of just 30 MW. RWEnPower has a five-year operation and maintenance contract for the plant.

Demand peaks

The maximum demand of the power system on mainland Greece in 2006 was approximately 10 000 MW, generated mainly by steam plants, fuelled with locally-mined lignite, and hydro stations. The Thessaloniki plant is one of five combined cycles built in recent years and was the first IPP to be licensed in September 2001. From the outset the IPP was designed as a merchant plant and bids to supply HTSO 24 hours in advance on the basis of availability and energy demand.

There are two demand peaks, of which the larger typically runs from the end of May to early September because of tourism and the high air-conditioning load. The winter peak lasts from mid- December through to early February, and is the result of a large number of heat pumps installed for space heating and for air-conditioning in industry and commercial premises.

The boiler of Thessaloniki Power’s plant, which is Greece’s first IPP
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During these periods the Thessaloniki IPP is run continuously in baseload during the daytime and at between 60-100 per cent load according to system demand during the night. For the rest of the year the plant is run from Monday to Saturday on full load from 0800 to 2200, and at 60 per cent over night, shutting down at midnight on Saturday for 30 hours.

In reality, the plant is despatched by the grid operator according to demand and the availability of other plants. In general, control rests with the station staff, but there may be critical times during the summer demand peak when HTSO can request auto generator control.

Plant design

The station is a two-shaft unit, with the gas turbine manufactured by GE and the steam turbine by Franco Tosi mounted on parallel axes in a single building and a CMI vertical heat recovery boiler outside. The gas turbine is the GE Frame 9FA+e rated at 258 MW ISO, with an exhaust temperature of 640 à‚°C, which is the highest of any gas turbine currently in production. Thus the boiler is one of the largest that CMI has made and is its 14th installation behind the Frame 9FA family of gas turbines.

The advantage of the vertical boiler to merchant operations is its lower thermal inertia and the consequently faster starting capability. Also, the tubes are horizontally supported, with the lowest ten m off the ground allowing room for a blow down tank to be mounted below. During a hot start any condensate created in the high pressure (HP) superheater during the initial purge sequence can be drained off naturally under gravity.

These high drainage capabilities, together with the flexible serpentine arrangement of the tubes, are the reasons why the vertical heat recovery boiler is well suited to cyclic operational duty. For this particular unit, CMI designed it with very low pinch points on the evaporators, resulting in a large heating surface area to maximize heat transfer as per the guaranteed heat balance established by the main contractor.

Thessaloniki is in a seismically active region and suffered a major earthquake measuring 6.8 on the Richter scale in July 1978. Thus, certain design features of th boiler take note of this and include a strengthened frame. In addition to the heavy structure, the deaerator is incorporated in the low pressure drum and the feedwater tank is mounted separately at a lower level on the side of the boiler frame.

The gas turbine exhaust duct and the HP superheater are part of a complete cold casing design with internal insulation. The transition from the circular section of the gas turbine duct to the rectangular section of the gas path through the boiler is completely in the horizontal plane and anchored to the ground. The cold casing of the vertical gas path is built up from it. This design is an upgrade from the general design of vertical boilers, where external insulation was still used in the inlet duct.

GE’s Frame 9FB gas turbine, mounted on a steel foundation with all auxiliary systems underneath
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In the vertical gas path, an expansion joint is installed to accommodate differential thermal expansion between the cold inlet duct positioned on the ground and the upper hot casing supported from the top of the boiler frame. However, more often these days this expansion joint is made redundant by a complete cold casing design up to the boiler stack, so that the casing of both the vertical and horizontal heat recovery boilers are both cold casing designs.

In other respects, it is a full natural circulation design without even start-up

pumps, which has been used for some time, while the vertical heat recovery boiler has been converted progressively from assisted to pure natural circulation.

The 12 heat transfer modules are factory-assembled and arranged as three parallel stacks of four. The HP output is 320.5 t/h at 124 bars, 567 à‚°C, with reheat at 26 bars 567 à‚°C and the low pressure (LP) at 4.8 bars, 290 à‚°C. The condensate pump serves as the LP feed pump and the feedwater tank feeds the HP and intermediate pressure (IP) steam paths.

The steam turbine is a two-cylinder design with combined HP and IP, and a double-flow LP cylinder with an underslung condenser. The cooling system is a closed circuit through a mechanical draft cooling tower, with ten cells.

Dual fuel capability

The gas comes via a pipeline from Russia, but there is also a liquefied natural gas terminal near Athens that imports gas from Libya as a second source of supply to cover variations in industrial and domestic demand. This balances supply so that there is always enough for the power plants.

For the Thessaloniki plant natural gas is the primary fuel, but there is provision for oil firing. The gas turbine runs on oil for half an hour every 15 days, which maintains its dual fuel capability.

The power plant went into commercial operation on 24 December, 2005 and is due to undergo the first hot gas path inspection at 8 000 EOH in late March 2007, which will include inspection and replacement of the combustion liners and transition pieces. Intake filters will also be changed at this time.

External factors

Autumn is one of the times of the year when the combined-cycle will run in mid load, with reduced running at night and shutdown on Sundays. However, the autumn of 2006 was particularly dry. This meant that there was insufficient rainfall to top up the levels of the hydro reservoirs, and therefore limiting production of hydroelectric power.

During a site visit at the Thessaloniki IPP on 24 November 2006, the plant had run continuously for two weeks and has since continued to do so over the following weekends. Further factors that will influence operations are the growing components of wind energy and the European Union’s (EU’s) carbon trading scheme as it influences the lignite fired plants, which are subject to the Large Plant Directive.

The combined-cycle has a NOx emissions limit that has effectively defined the 60 per cent technical minimum operating limit (for overnight operation) when the efficiency of the plant falls to about 52 per cent, as against 56.6 per cent at full load.

According to Thessaloniki Power’s plant director George Kaltsas, emissions have been within limits at all stages of operation. This suggests that there could be scope for carbon trading under the EU regime, offsetting baseloaded lignite fired plants powered by locally mined fuel, with higher emissions and lower efficiency but also lower operating costs.

The plant had its first bore scope inspection outage at the end of May 2006 and again in October, and shortly after that there was a short forced outage to fix an issue in balance of plant. However, since then the plant had been operating continuously as up to the time of the site visit on 24 November 2006. At noon on that day the air conditions were 1010 mbar at 15 à‚°C. The gross electrical output was 380 MW, of which 243 MW was from the gas turbine and 137 MW from the steam turbine.

Steam conditions were 265 t/h, at 114 bars, 564 à‚°C, and with reheat to 564 à‚°C boosted by IP steam flow to 301 t/h, at 23.2 bars. LP steam output was 31.2 t/h, at 3.3 bars, 280 à‚°C.

This would be a typical operating condition at this time of year, except that because of the hydropower shortage, operation was continuously at baseload during the daytime and minimum load at night.

Off-peak operation

In a typical off-peak operation with a 30-hour shutdown at weekends, the plant starts every Monday morning, reaching the minimum combined-cycle load of 240 MW in three hours. The gas turbine is started with a static converter device operating through the generator. Initially it is held at 600 revolutions per minute (rpm) to purge the system and then decelerated to a firing speed of 400 rpm before being accelerated to synchronous speed. The whole operation takes 25 minutes.

As the gas turbine loads, the boiler, which has been kept warm by the stack damper over night, starts to send steam to the turbine, which reaches synchronization in an hour. It is then loaded at a rate of about 1 MW/min to bring the total combined cycle output to 240 MW, which must be reached by 0700. Thereafter the plant can be ramped up at 12 MW/min to the full load as required by the dispatcher.

Opening IPP floodgates

Thessaloniki is the first of several ind pendent power projects planned for Greece. There is already one peaking plant with 150 MW of simple cycle gas turbines, and a 200 MW cogeneration scheme is under construction at an aluminium refinery. But with one year’s operation behind it the operators are very pleased with the performance achieved.

The Thessaloniki IPP, in particular the vertical boiler, has been designed for cyclic duty with many stops and starts, as well as part-load operation to satisfy power demand on the new Greek merchant power market.

However, it has been a pleasant surprise to the owners that the unseasonally dry weather, which affected hydropower output has created opportunities to extend baseload operation beyond what was originally planned.

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