Vado Ligure unit 1 rejuvenated: from sixties coal plant to CCGT1 November 2007
On 9 July 2007 – in time for the summer peak load – unit 1 of Tirreno Power's 36 year old oil/coal fuelled Vado Ligure plant, originally rated at 320 MWe, began its new life as a modern 780 MWe combined cycle plant.
Located near Savona, on the coast south west of Genoa, Italy, the Vado Ligure power station (Figure 1) originally consisted of four 320 MW units, commissioned during 1970/71, designed to burn residual oil and coal. The station, located on a 42 ha site, was initially equipped for oil, but in the late 1970s was equipped to burn coal. Coal is delivered by ship and carried by enclosed conveyors to the storage pile and to the power plant. Oil was supplied through a pipeline from a dock located in a nearby industrial area.
Units 1 and 2 have been unable to burn coal in recent years due to tightening emissions limits and it was decided to repower unit 1 as a combined cycle plant (Figures 2, 3 and 4). Units 3 and 4, meanwhile, have been extensively upgraded to permit continued baseload coal burning while still meeting emission limits regulations.
The contract for the unit 1 repowering was signed between Tirreno Power (Figure 5) and Ansaldo Energia in March 2005, after an international tender issued during spring 2004.
The award was a turnkey EPC contract, based on a schedule of 27 months.
Site work started in June 2005, with demolition of old buildings. Main foundation works lasted from October 2005 (HRSG area) to April 2006 (gas turbine No 2 area). Limited space on the site made good co-ordination between the various contractors (civil, EPC, etc) very important, with Tirreno making available areas for storage of materials and for pre-assembly and prefabrication activities elsewhere on the power station site.
Erection of HRSG No 1 started in March 2006, while the HRSG No 2 hydro test was completed at the end of that year. Delivery to the site of gas turbines and generators was completed in June 2006 and first firing of GT No 1 was achieved at the beginning of March 2007 and of GT No 2 on 10 May 2007.
Ansaldo was also awarded a 12 year long term service agreement covering the gas turbines, plus their auxiliaries and generators.
Particular attention has been paid to the formulation of contractual obligations regarding the availability guarantee and the calculation of liquidated damages and incentives. The parties agreed in fact that the resulting amount of the liquidated damages and incentives, calculated on the basis of the combined performance levels of the two gas turbines, should be a reasonable estimate of customer's probable losses/profits and not a simple penalty/bonus for the contractor.
In addition Ansaldo and ABB have developed an innovative control system making the new plant one of the most advanced CCGT stations in Italy.
The new plant has been designed to limit flue gas emissions to the atmosphere to the following levels:
These limits are based on average hourly concentration, normalised to the reference condition for O2; they apply at any plant operating condition in the range from low load to 100% of the gas turbine nominal power. And all limits are to be attained with the gas turbine operating in dry mode, without injection of steam or water.
Noise is a serious constraint for this combined cycle plant, as the nearest house is only about 100 m away (Figure 3). The noise limits are determined by Italian Law 447/95, regional law n.12 (dated 20.03.1998) and approvals by the local municipalities. The requirements for differential noise input, compared with existing background noise, are 5 dB(A) during the day and 3 dB(A) during the night, while the more stringent absolute acoustic emission limit is 50 dB(A).
The Vado Ligure units, as originally built, are pretty much independent of each other, each consisting of one boiler plus auxiliaries, one steam turbine with condenser, independent steam/water cycle and independent connection to the grid. During the repowering work units 3 and 4 have remained in full operation.
The original boilers are Ansaldo/ Babcock & Wilcox units, subcritical once-through with single reheat. The unit 1 and 2 furnaces were pressurised, while the unit 3 and 4 furnaces have been modified for balanced draft operation with a negative furnace pressure (through the addition of a 2500 kW induced draft fan).
The improvements made over the years to unit 3 and 4 include low NOx burners, FGD systems for SO2 control, SCRs for NOx control and ESP upgrades. OFA and coal reburn were also added on unit 4.
The turbines are tandem compound units manufactured by Ansaldo, nominally rated at 320 MW, with 33.5 inch last stage blades, reheater and final condensation. There is no turbine start-up bypass system.
Condensers, located under the turbines, are of the single pass type with two shells and separate water boxes. They are seawater cooled with a once through circulating water system supplied by four 100% capacity vertical single stage pumps. Water is pumped through four submerged offshore intake pipes to the condensers and conveyed back to the sea through a closed concrete discharge tunnel.
The new layout
The new combined cycle plant is of the multishaft type, with two gas turbines, each with an HRSG, plus one steam turbine installed on the foundations of the old unit 1 steam turbine in the existing turbine hall.
It is worth noting that a brownfield project may be much more demanding than a greenfield one. But, as is well known, in Italy the population density is very high and it is not easy to find new opportunities for greenfield projects.
Due to the constraints arising from the existing buildings, the new combined cycle modules have been built at right angles to each other (Figures 4, 6 and 7).
One gas turbine with its HRSG has been installed where the old unit 1 boiler and ESP were, previously dismantled by Tirreno Power. Their axis is parallel to the steam turbine. The other gas turbine HRSG combination forms a right angle with the first one, and is positioned on the location of the old workshop/administrative building, which was partially demolished by Ansaldo Energia. The consequence of this solution is that the stacks of the two HRSGs are adjacent to one another and structurally tied together (see Figure 4).
Both HRSGs and stacks are installed inside an architecturally designed weather proof structure, which aims to minimise visual impact. Each gas turbine with its generator, together with control system, is installed in its own building, with electrical equipment adjacent to it.
On the northern side of the steam turbine hall is the new electrical substation, which is of the GIS type, linked to the transformers by three phase 380 kV XLPE cables. The gas station is located at a distance of about 500 m from the new plant.
A particular effort was made to optimise the layout, to partially recover existing equipment where possible, to avoid interference with the functionality of units 3 and 4, and to verify the capabilities of existing auxiliary systems (auxiliary steam, compressed air, service water, demineralised water, etc) in meeting the requirements of the combined cycle units.
The new gas turbines, steam turbine and generators, were manufactured by Ansaldo Energia and transported by ship to the dock close to the power station.
The two new gas turbines are of the V94.3A2 type. Their air intakes are equipped with anti-icing systems and fogging for inlet air cooling. The associated gas turbine generators are of the air cooled, WY23Z-109, type.
The steam turbine body is composed of two sections: one IP-HP combined section that replaces the old one and fits the existing pedestals and foundation and one new inner LP block installed in the existing LP outer casing. The blading is of the reaction type.
Other parts of the old steam turbine were recovered, such as bearing pedestals, the cross-over pipe between the LP and IP sections, turning gear, lube oil system, steam seal system and gland air steam condenser.
The original steam turbine generator has been re-used, following a general overhaul by Ansaldo Energia and rotor repairs. New auxiliaries were installed, including excitation system, terminal and neutral point cubicle, transformers, instrumentation, etc.
The two HRSGs are of the horizontal type, with three levels of pressure.
The HRSG stacks are 90 m high and equipped with exhaust gas monitoring systems. The steam condenser was subjected to a major overhaul and a new cleaning system provided.
Besides the main equipment, the new supply also included main auxiliary systems, such as feedwater, closed cooling system, main steam, circulating water, sampling, chemical injection, natural gas system.
Other mechanical equipment was totally or partially re-used after a general overhaul. This included condensate extraction pumps, polishing system, vacuum system, seawater piping, and seawater travelling screen.
On the electrical side the supply essentially consisted of new equipment, including the installation of three 300 MVA, 380 kV step-up transformers and 400 kV GIS for the connection of the generator units with the Terna substation. The GIS includes one bay for Terna substation connection, two bays for gas turbine generator connection, one bay for steam turbine generator connection and one set of three phase main busbars with PT's and earthing switch.
Distributed control system
The DCS is based on ABB IndustrialIT architecture with AC 800F controllers, I/O S800 units linked via Profibus DP, smart devices (AE, E/P, MCC, PC/MV) and field instrumentation integrated via field bus according Profibus DP e PA profiles.
Ansaldo Energia/ABB GTCMPS (Figure 8) and STCS packages complete the supply for the gas and steam turbine control respectively in an integrated architecture.
ABB and Ansaldo have jointly developed the relevant control systems, taking into account all the specific requirements of the V94.3A gas turbines.
The human–machine interface is based on ABB's Power Generation Portal (PGP), which includes the following software packages: Load Profiles Coordinator; Automatic Voltage Regulator System; Plant Assessment; Sensor Validator; Computerised Diagnostic System; and Turbine and Boiler Stress Evaluation.
Load Profile Coordinator
The Load Profile Coordinator (LPC) is a decision support tool, working in the PGP environment, devoted to managing easily the daily operational schedules, allowing operation personnel to monitor the actual energy production versus the contractual level.
The tool is provided with suitable alarms, logs and archiving files to effectively operate in a dynamic market.
The operation personnel can change the plans according to the actual operating conditions and/or in response to additional requests from the dispatcher.
From an operational point of view the package sends the planned power set point to the DCS and recovers automatically the energy imbalances.
Voltage/Reactive Power Regulator System
The Voltage/Reactive Power Regulator System is a package devoted to the control of the bus voltage and reactive power at the plant level, fully integrated into the DCS architecture, with the aim of harmonising plant behaviour with the needs of the rest of the network.
The architecture is based on three levels (see Figure 9):
• AVR (Automatic Voltage Regulator) – controls the voltage of the unit;
• PQR (Reactive Power Regulator) – controls the reactive power of the unit, according to the set point of the RVR;
• RVR (Regional Voltage Regulator) – establishes the reactive power set points according to the pilot bus voltage of the network.
The Performance Calculation continuously compares actual performance indexes (heat rate, stage efficiencies etc), calculated according to international standards such as ASME and IEC with reference ones, both for equipment and for the plant. The system is also able to perform loss analysis to identify the impact on the power supply and/or heat rate of the identified deviations.
This tool provides continuous and on line monitoring of sensor and instrumentation status by comparing the actual value of the field measurements (temperatures, pressures, vibrations, etc) with the package estimated ones, based on a reference data set and generating alarms in the event of significant deviations. Intuitive visualisation and an easily extendible data set make the tool a powerful support for plant maintenance people.
Computerised Diagnostic System
The Computerised Diagnostic System (Figure 10) is a support device that permits continuous equipment maintenance information management, with the benefit that all the information is cumulatively stored and continuously updated. It also gives statistical information about the deterioration of equipment and the plant systems.
The most important information (eg, running hours, diagnostic warnings, etc) are sent to the Computerised Maintenance Management System (CMMS), provided by a third party (SAP PM), for further actions.
The tool also includes checking of the on-off field-bus actuators, comparing of torques with nominals and checking the energy consumption of each operation.
Turbine and Boiler Stress Evaluator (Fig 11)
These functions have assumed new importance due to the cycling operations now required by the power market. As a consequence the indication of a true cost of a certain load profile is not only related to fuel and personnel, but also to equipment degradation.
The system monitors the status of the equipment, in terms of thermo-mechanical stresses and creep conditions. According to current physical conditions and historical data, life consumption is calculated and compared with theoretical figures. In particular, HRSG life consumption calculations are performed in accordance with the most recent European standard, EN 12952.