Completing Mexico's Monterrey III ahead of schedule

5 November 2002

Iberdrola Energía Monterrey's new 1000 MWe combined cycle plant has set new benchmarks for project implementation in the Mexican power sector.

With Mexico enjoying high rates of economic growth and development, electricity generation capacity and natural gas resources are straining under unprecedented demand. In the city of Monterrey the electricity demand growth rate has been as high as 6.8 per cent, due to a rapid rise in manufacturing and increased commercial and residential consumption, while much of Mexico's north is experiencing an average electricity demand growth rate of 6 per cent per year. Overall, it is projected that Mexico will install 28.8 GW of new capacity between now and 2011 (about $23 billion worth), taking the country's total installed capacity to 63.2 GW.

In addition, the natural gas infrastructure continues to expand vigorously as the country strives to lessen its dependence on hydro and coal-generated electricity. A projection by Pemex (January 2002), predicts that the region's natural gas consumption will reach 7.9 billion cubic feet per day by 2009, twice its 1999 level, with several transborder natural gas projects, including LNG schemes, being planned and developed.

The 1000 MWe Monterrey III plant (pictured above), owned by Iberdrola Energía Monterrey, consists of four Alstom KA24-1 ICS single-shaft combined cycle power blocks, each with a GT24 gas turbine, HRSG, steam turbine and generator. It is located about 15 km from the city of Monterrey itself, in Pesqueria, Nuevo León. The plant has been built adjacent to Monterrey II, a 500 MW combined cycle plant also equipped with the GT24 technology.

Ingredients of success

Site mobilisation, soil survey, building permits, component transport/import and civil works were completed for all four blocks within 16 months of the notice to proceed. Within a further six months, the first three blocks had been synchronised.

The order for Monterrey III was placed by Iberdrola Energía Monterrey, SA de CV, a special purpose company established by Iberdrola, Spain. Alstom was responsible for most of the engineering, procurement and construction.

The first three blocks entered commercial operation in spring 2002, ahead of schedule, with a total construction time of 24 months. The fourth block will be commercially operational in spring 2003.

The project was tendered to independent power producers (IPP) on a build-own-operate basis by Mexico's Federal Electricity Commission (CFE), the public utility.

Iberdrola signed a 25-year contract with CFE, and another long term agreement with companies in the private sector, Grupo Alfa-Pegi, Apasco, Femsa and other clients to deliver electricity. The first two units serve the public utility, and the second two the private customers.

Monterrey III is a turnkey plant having many interfaces with the client's contractors, who are under the leadership of Iberinco, the engineering group of Iberdrola. From the start of the project, Alstom realised that this would be a high-risk area and specified the parameters of all interfacing. This resulted in excellent co-operation between the two main players, Alstom and Iberinco.

The relationship between client and contractor was very positive, one of the many contributing factors being the amount of time the project management team spent on site, co-ordinating efforts between site team and client.

Also, because of the great distances involved and the importance of the correct equipment arriving on site at a specified time, a system was set up by the project team to ensure that the precise cargo load on all shipments was predetermined. This ensured that the construction site knew exactly what to expect in each shipment. This in turn helped to keep installation ahead of schedule, while at the same time achieving cost savings on transport, customs-transit and construction.

Working to a guaranteed delivery schedule of 24 months for the first two units and a 25.5 month guarantee for the third unit, the first two were handed over six days early while the third unit was commercial 16 days early. The intensive contract management, as well as the good co-operation between Alstom, its subcontractors and the highly experienced site team, which pursued strict progress supervision, contributed to this achievement.

The 7-day reliability run was successfully completed on all three machines at the first attempt. Since PAC (provisional acceptance certificate) on 25 March 2002 for the first two units and 29 April 2002 for the third unit, all machines have been operating in a continuous baseload regime.

A particular feature of the plant is its compactness, much of which stems from the use of single shaft units. These take up much less space than other types of plant.

The arrangement of the power plant has been adaptated to the local landscape, with a 4 m high step between the water treatment plant, switchyard and power islands.

Monterrey III has a 2x2 arrangement, which will allow it to be split into two separate plants, should that be legally required at some point in the future (eg for sale to two different buyers).

Another feature is the length of the cooling tower facility, designed for optimal integration with the landscape, which also helps meet the overall noise criteria.

The electricity supplied to the grid is three-phase 60 Hz, at a nominal voltage of 400 kV (units 1-3) and 115kV (unit 4) after the main step up transformer.

For each individual power block, the plant can be dispatched with a power factor in the range between 0.85 lagging to 0.90 leading by the operator, measured at the generator terminals.

The plant has a 115 kV and a 400 kV switchyard connected via an auto-transformer. This ensures that the 115 kV line, which serves industrial clients, can be fed from the fourth unit and/or from the 400 kV grid. Any portion of the fourth unit's output that is not distributed to industrial clients on the 115 kV line can be fed into the 400 kV system. And, if the 115 kV grid is not available, it is even possible for all the output from the fourth unit to be distributed via the 400 kV line. This arrangement makes the facility very flexible.

The plant is designed to run at continuous part-load or baseload operation, with the possibility of daily start-up and shut-down within an ambient temperature range of 3.5°C to 44.2°C and with natural gas as fuel.

The reference ambient conditions are 39°C, 0.968 bara, and 33 per cent RH. Under these reference conditions, each of the four combined cycle blocks has a generating capacity of 250 MWe.

The overall plant process control system for the plant is realised in Advant Power and is hierarchically structured.

Project organisation

Alstom Power Monterrey III SA de CV was established, in addition to the existing local Aslstom organisation, as a special purpose company for several projects awarded in Mexico. The company is responsible for project execution and developing local competence in the handling of EPC contracts.

The Mexican state utility, CFE, initiated international tenders in the mid-1990s for combined cycle power plants.

After the previous "BLT" (build-lease-transfer) scheme, the new projects were to be offered to CFE on a build-operate-own basis, which includes financing, equity, fuel supply, sub station and transmission line provision and supply of a turnkey power plant to the bidder/owner.

Alstom Power Monterrey III SA de CV is responsible as a main contractor for signing locally the EPC contract with the plant owner, which must be registered in Mexico. The responsibilities of Alstom Power Monterrey III include:

• minimum 25 per cent local procurement;

• local project management with single point interface between plant owner and off-taker, CFE;

• management of project engineering, procurement and construction contracts;

• local construction permitting;

• implementing of contractor's insurance including marine cargo and builder's all risk;

• management of offshore and onshore equipment delivery, including transport plus payments for import duties, taxes and fees;

• complete site management during construction, training of owner's operators and provision of all the local construction and operating permits;

• setting up of a well-staffed office for handling all commercial issues including personnel recruitment.

Overall, within the scope of the EPC contract, Alstom Power Monterrey III, handles all critical events during construction, commissioning, synchronisation and commercial operation of the plant.

Gas turbine and generator

The GT24 type gas turbine is of single shaft design, running at 60 Hz. It is rigidly coupled to the generator shaft. There are five turbine stages and 22 compressor stages. Heat input is performed by two annular combustion chambers (EV+SEV burners), applying the sequential combustion principle.

Air is taken through the two stage air intake system and compressed in the compressor.

The inlet air is cooled by passing it via the wet evaporative cooler medium. This reduces the air temperature by evaporation, increasing air density and thus power output. Because of the geographical position of Monterrey and the ambient conditions prevailing there, the evaporative cooler can be operated most of the time, which is advantageous.

After the last compressor stage the air flows around the annular combustor, to cool it and after that enters the EV burners where it is mixed with the fuel and burnt. The hot gases then flow through the one stage, high-pressure turbine into the annular SEV combustor. To reheat the gases, additional fuel is injected through spray nozzles into the annular SEV combustor and burns with the gases coming from the turbine stage. The reheated gases flow through the low pressure turbine stages 1 - 4 and the exhaust diffuser into the heat recovery steam generator.

The airflow through the gas turbine is controlled by three variable guide vane rows, which are part of the first three compressor stages. By changing the angular position of these rows, the air flow is adjusted because of the changed flow cross section. Together with fuel injection adjustment, the exhaust gas temperature can be controlled during part-load operation.

The compressor is equipped with blow-off valves at three stages. Excess air is blown off into the exhaust system during start-up and shut-down. This prevents rotating stall, protects the compressor from damage and reduces start-up power.

For cooling and sealing purposes, air is drawn off from the compressor at several stages. This air flow is partly cooled by two once-through coolers which are fed with HP feedwater from the HRSG system.

If the ambient conditions pass critical values for the building of ice in the first compressor stages, the anti icing system is switched on automatically. Hot air is extracted from the GT compressor and guided into the GT air intake. During this mode the power output and efficiency of the unit are reduced.

The second generation, B type, GT24 machines at Monterrey III incorporate modifications that have been introduced across the GT24/26 fleet in response to technical issues that arose when the B technology was introduced commercially in 1999/2000.

The generator rotor is directly coupled to the gas turbine rotor. The footplate mounted 2 pole 3-phase synchronous generator is of TEWAC (totally enclosed water-air cooled) design. This means the primary cooling media inside the generator is air and the secondary cooling media is water.

The gas turbine requires an external start-up source and the generator is used as a synchronous starting motor fed by a static frequency converter.

Only two static frequency converters are installed for the total of four gas turbine units, with a change over facility for redundancy. This design ensures that any of the four units can be started by one of the two starting devices.

During the start-up period the starting energy is provided by the HV grid across the generator-step-up transformer.

Steam turbine and SSS clutch

The steam turbine is of two casing design, with a single flow low-pressure exhaust. The high pressure section is a geared turbine. The IP/LP turbine section operates at generator speed. The clutch between the steam turbine and the generator is of a self synchronising design.

The barrel type high pressure turbine section has a set of 10 stages of impulsive blading. The combined IP/LP turbine section has a welded rotor of the drum type. The blade path, with 22 stages of reaction type blading, enables high efficiency under full and part load conditions.

The live steam enters the HP turbine through one stop and one control valve. After the expansion in the impulse blading down to reheat pressure, the steam leaves the turbine to the reheat section of the HRSG. The reheated steam is admitted to the IP/LP turbine part via one intercept stop valve. An LP steam admission upstream of the LP turbine blading provides an additional steam flow, admitted through a control and stop valve.

The speed reduction gear between the HP section and the IP/LP part is of a single helical design. Each side of the gear is equipped with an intermediate shaft to the appropriate turbine sections to guarantee safe dynamic behaviour of the turboset.

The SSS clutch permits the steam turbine to be accelerated and connected to the generator already being driven by the GT. The clutch engages automatically as soon as the speed of the ST tends to overtake that of the generator. The clutch automatically disengages as soon as the ST tends to decelerate with respect to the generator.

  For sealing of the steam turbine, live steam is supplied to glands during start-up and low load operation. A desuperheater fed with condensate controls the sealing steam temperature for the HP/IP. A second desuperheater reduces the gland steam temperature for the LP turbine glands. The suction side of the steam turbine glands is kept at a pressure slightly below atmospheric by a gland steam exhaust fan.

At increased steam turbine load, leakage steam from the HP turbine replaces the steam supply from the HP live steam and excessive gland steam will be discharged to the condenser through the condenser through the gland steam dump valve.

HRSG and bypass

The once through heat recovery steam generator produces superheated steam at two pressure levels: HP and LP. The boiler comprises an LP economiser feeding into the LP drum, an LP evaporator operated in natural circulation mode, and an LP superheater. The HP circuit of the boiler is designed for once through operation with economiser, evaporator, and superheater arranged in series and the reheater located in-between the HP superheater sections.

The LP system is fed directly with condensate from the condenser hotwell. In normal operation condensate is preheated in the LP economiser before entering the LP drum. The LP feedwater control valve is situated between the LP economiser and the drum to prevent steaming in the economiser. For feedwater preheating purposes, hot water is recirculated from the LP economiser. The HP circuit is fed with water from the LP drum.

The RH is fed with superheated steam leaving the HP turbine and returns at full temperature to the IP turbine. To control the RH outlet temperature, an attemperator is situated at the reheater outlet.

The HP system is fed by two feedwater pumps (units 1-3) and one feedwater pump in the case of unit 4. During full load operation, variation of HP feedwater flow is the only means used for control of live steam temperature. Additionally, a HP desuperheater station is installed at the HP superheater outlet for limiting HP live steam temperature during start-up, transients and part load.

A water/steam separator is situated at the inlet of the HP superheater. In normal once through mode it receives superheated steam from the preceding HP boiler section. During start-up, however, it receives a two-phase mixture and separates the saturated water for recirculation via the LP drum. The water recirculation to the LP drum is established by the pressure difference between HP separator and LP drum.

During very early stages of start - up, when said pressure difference is not sufficient to establish recirculation to the LP drum, the water is rejected through the blow down ensuring minimum flow through the HP economiser

For reasons of steam quality, the separated water may be blown down for removal of contaminations in the water/steam cycle by overfeeding and/or load reduction.

To increase operation and start-up flexibility of the plant, there is a 100 per cent steam bypass system which operates during start-up, shut down or load rejections of the steam turbine.

The steam bypass system is designed to handle the entire steam production at full pressure under all ambient conditions, for a prolonged period. It consists of fast closing stop valves and steam pressure reducing valves with integrated water injection for the HP and LP live steam line. Injection water for desuperheating of the steam is taken from the main condensate line. The reheater section is designed to run dry during bypass operation.

Cooling systems

A common forced draught wet cooling tower system transfers the waste heat of the water/steam cycle to the atmosphere. One 100 per cent main cooling water pump per unit supplies the cold water from the cooling tower basin to the main condenser and in parallel to the intercoolers of the closed cooling water (CCW) system. The main cooling water systems consist of 5x100 per cent pumps for all four units, of which one pump is on standby in case of a pump failure.

Cooling tower make-up from a water source is provided to compensate for the loss of cooling water due to evaporation and blow-down.

The CCW system ensures the secondary cooling of the generator, lube oil and other consumers. Two 100 per cent capacity circulating pumps are provided for this system. The heat picked up in the CCW is dissipated to the main cooling water system via two 100 per cent capacity water-to-water heat exchangers.

The main condenser is equipped with a sponge ball cleaning system to avoid biofouling and scaling of the condenser tubes.

Water supply system

Sewage water is supplied to the power plant, clarified and filtered in the pre-treatment plant. A further step in the osmosis plant provides water to be used for the evaporative cooler and fire fighting.

Two demineralisation plants with anion/cation exchanger trains, sized for continuous blowdown operation, ensure the supply of demineralised water for the facility. Each plant is considered as one module, serving treated water to two units. The treated water is stored in a demineralised water tank.

Demineralised water pumps forward the treated water from the tank to the consumers. The treated water is sprayed into the condenser flashbox and replaces the water lost due to blowdown and leakage in the water/steam cycle.

Fuel gas supply

The fuel gas is delivered to the plant via pipelines. Due to the possible wide range of fuel gas supply pressure and quality conditions, it has to be treated/conditioned before it can be fed to the GT fuel gas blocks. The fuel treatment system consists of the following main components: dew point heater; metering station; liquid and dust separator with condensate skid; 5x100 per cent gas compressors in total (one unit on standby); pressure reducing station; and fine filter.

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