Italy's dash from oil to gas: the case of La Casella

Italy is currently in the throes of a very extensive programme for the conversion to combined cycle of existing conventional oil-fired units. At the time of writing, contracts had been awarded to Enelpower for the conversion to combined cycle of 24 existing units. After repowering, the total generating capacity of these plants will amount to some 9000 MWe. Enelpower is the company of the Enel Group that inherited the engineering and construction activities of ENEL and was awarded these contracts on the strength of its experience with such projects and its consequent ability to achieve minimum costs and completion times.

Ten of the 24 units contracted so far are owned by Enel Produzione, while the others are owned by private investors such as Endesa Italia and Edipower (who have purchased the generating companies that have been spun off from the Enel Group), or by municipalities like AEM Milan or ASM Brescia.

Other similar repowering projects are expected to be developed in the near future, increasing the total capacity of converted plants to over 12 000 MW. The total installed capacity in Italy is around 60 000 MWe, the principal owners being Enel Produzione (40 000 MW), Edipower (12 000 MW), Endesa (5 400 MW), and Interpower (2 600 MW). For the moment, Interpower remains part of the Enel group but is in the process of being sold.

The considerations behind and the rationale for the Italian repowering projects can be summarised as follows:

• generating capacity in Italy, at present, is sufficient to satisfy the demand;

• most of the existing plants are obsolete, in terms of both their environmental performance and competitiveness;

• the development of new sites in Italy is problematic due to the environmental sensitivities of public opinion;

• the cost of a conversion project is 25-30 per cent lower than a green field project, taking into account the widespread re-use of existing equipment and infrastructures, such as auxiliary systems, buildings, cooling water systems, fuel systems and transmission lines. At the same time the penalty in efficiency loss due to the re-use of existing equipment not optimised for the new conditions is very limited.

Obviously, the engineering and construction aspects of repowering are much more complex than for new build because of the constraints arising from the existing equipment and plant lay-out; every project has to be tailored to the specific site requirements.

All of the Enelpower repowering projects are based on the concept of removing the existing boiler and substituting for it a 250 MW gas turbine with its associated heat recovery steam generator. The steam produced is used to feed the existing steam turbine.

Several schemes and lay-outs are being used, depending on the specific project requirements (extent to which existing equipment is to be re-used, the need to maximise generation during the repowering works, space availability, etc).

The repowered power plants also undergo architectural restyling, with the objective of upgrading the landscape of the plant surroundings and improving working conditions at the plant.

In the north of Italy the natural gas comes from Russia, while in the south, Algeria is the source.

The NOx emission limit for gas turbine plants in Italy is 50 mg/Nm3 and the gas turbine combustion systems adopted in the repowered plants (in the case of both GE and Siemens turbines) are of the dry low NOx type.

The La Casella case

A typical example of a repowering project, and indeed the lead plant, is the conversion of Enel Produzione’s La Casella power plant from oil to gas, with Enelpower in the EPC role.

The main characeristics of the original La Casella plant can be summarised as follows:

• 4×304 MW (net) oil fired units;

• net efficiency of 40 per cent LHV;

• boilers: supplied by Ansaldo, frontal firing, once through type, rated 1050 t/h;

• steam turbines: supplied by Franco Tosi, rated 320 MW, tandem compound, two casings (1 HP, MP) 1 LP double flow;

• generator supplied by Marelli, rated at 370 MVA;

• cooling system: direct type, with water taken from the Po river;

• first synchronisation dates: unit 1 – July 1971, unit 2 – December 1971, unit 3 – June 1971, unit 4 – 1972;

• fuel: heavy fuel oil;

• emissions: SO2, 400 mg/Nm3; particulates, 50 mg/Nm3; and NOx, 200 mg/Nm3 (dry, 15% O2).

For the repowering project, the basic approach was to accommodate the new gas turbines and their respective generators in the area where the ESP and flue gas ducts were installed prior to the conversion.

On the repowered units, the gas turbine exhausts discharge directly into the HRSGs. The HRSGs, supplied by Ansaldo Caldaie, are of the vertical type, installed into the steel structures of the old boilers (only the pressure parts were demolished). The steel structure was appropriately reinforced to cope with the new loads and the new lay-out. New chimneys are installed above the HRSGs to discharge the flue gas into the atmosphere. The old chimneys will be demolished.

The HRSGs (with three pressure levels pus reheat) were tailor-made to fit the existing structures.

The gas turbines for La Casella were procured without generators. For two of the units, refurbished generators from another plant have been used. For the third unit a new generator was procured.

The original steam turbines and their associated generators have been reused from the old plant. In the steam turbine, the HP section has been modified. The governing stage has been removed and the steam path altered, to obtain high efficiency under the new operating conditions.

The condenser has been modified, making use of the space created by removing the LP heaters installed at the condenser neck to install the bypass system connection.

So far, the gas conversion process is only being carried out on three of the four La Casella units. But all the documentation necessary to achieve the necessary permissions and authorisations to repower unit 4 has been submitted to the authorities.

The main data for the three repowered La Casella units is as follows:

• installed capacity: 3×380 MW;

• efficiency: 56 per cent;

• new gas turbines from Ansaldo/Siemens, type V94.3A(2) (254 MW);

• generators for the gas turbines supplied by Alstom;

• fuel: natural gas (from Russia, as the plant is located in the north of the country);

• emissions: NOx, 50 mg/Nm3 (dry, 15% O2); particulates and SO2: negligible.

Key dates for the La Casella repowering prject include the following:

• award of the gas turbine contract (Ansaldo/Siemens), February 2000;

• gas turbine synchronisation for the first unit, October 2002;

• steam turbine synchronisation for the first unit, December 2002;

• gas turbine synchronisation for the second unit, April 2003; and