Athens-based power plant contractor METKA is building a 320 MWe cogeneration plant for Endesa Hellas which will provide process steam to a major alumina and aluminium production facility on the coast of Greece. Paul Smith describes the design philosophy and the operational flexibility built into the plant to allow it to meet varying site energy needs.

Aluminium of Greece (AOG) is a major integrated alumina and aluminium production complex at Agios Nikolaos on the north coast of the Gulf of Corinth. The production of the 730,000 tonnes per year of alumina (aluminium oxide) from bauxite requires significant quantities of process steam. The substantial local deposits of Bauxite, including mines operated by the subsidiary company Delfi-Distomon Bauxite SA, as well as imports form the supply. The complex produces 170,000 tonnes of aluminium each year from alumina in an electrolytic process that consumes large amounts of electricity.

This paper describes the AOG CHP project which METKA SA is executing on an engineering-procurement-construction (EPC) contract basis at Agios Nikolaos. The new CHP plant, owned by Endesa Hellas, will produce steam for the alumina production process and its electricity will normally be exported to the national grid. In back-up mode, the CHP plant will supply electricity within the site to AOG for use in the aluminium production process.


View of the project under construction, April 2007
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The AOG production complex is a rare example of production facilities for alumina and aluminium being at the same site. More commonly, alumina production plants (refineries) are located near bauxite deposits, while aluminium production plants (smelters) are located near major power generating plants. The co-existence of these elements at Agios Nikolaos provides s a significant competitive advantage. The investment in the new CHP plant enables the overall energy demands of these integrated facilities to be fulfilled in an efficient and environmentally friendly manner.

Project execution methodology

Following consideration of the various options, the client elected to carry out the cogeneration project as a single EPC contract on a lump-sum, turnkey basis. The primary benefit of this from the customer perspective was a single point of contact for the entire execution of the project and reduction of potential project risks. METKA was subsequently selected as the EPC contractor for the project following an extensive international competitive bidding procedure. The contract was signed in July 2005.

METKA’s scope of supply as EPC contractor includes all required design and engineering services, procurement of all equipment and materials, and construction, testing and commissioning of the complete plant. In addition to the main CHP plant itself, the scope includes all required civil works, connection to the natural gas network and all electrical scope, including the 150 kV substation and grid interconnection.


View of the AOG production complex in April 2007. The cogeneration plant, under construction, is on the right-hand side
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Within the context of the EPC contract, METKA is fully responsible for the design and engineering of the new CHP plant. METKA carried out part of the engineering work directly with in-house resources (overall engineering supervision plus detailed design responsibility for the civil works and electrical part) and outsourced other engineering services. Based on a successful collaboration for an earlier combined cycle project at Unit 5 of the thermal power station at Lavrio, METKA selected Empresarios Agrupados of Spain to provide engineering services for the main part of the CHP plant.

METKA’s in-house resources exclusively carried out procurement for the project, whose short schedule meant that all major procurement activity had to be fast-tracked. This became especially important to the overall success of the project due to the tightening global power equipment supply market. The suppliers came mainly from the company’s existing vendor database, supplemented with a few others that were audited for quality and experience during the procurement process.

METKA has a particularly strong background in the construction aspects of major power plant projects, having carried out a very large proportion of the erection works for the existing power plants in Greece. METKA’s dedicated site team managed the construction on-site for the CHP plant (see photograph above) and assigned specialist supervisors for each discipline: mechanical, electrical, instrumentation and civil. Qualified subcontractors provided skilled and semi-skilled construction labour. Similarly, METKA also carried out the commissioning directly, with some specialist support from main equipment suppliers.

Basic design considerations

The production complex at Agios Nikolaos operates throughout the year, with the alumina production process continuously using HP and LP steam. The CHP plant is therefore designed to deliver HP and MP steam to AOG continuously. The HP steam is used within the process of extracting alumina from bauxite (referred to as digestion). The MP steam provided to AOG by the CHP plant is reduced to LP for use in the alumina plant evaporators and other LP steam consumers.

Fundamental requirements for the design included the following:

  • High reliability of the steam supply to the alumina process plant. This is critical since interruption of the steam supply can, in certain cases, lead to severe damage to process plant and to a shutdown lasting several months.
  • Operational flexibility. The CHP plant must be able to meet the electricity and steam demands of the integrated alumina/aluminium complex under a wide range of operational modes.
  • Steam quality. This is critical for the process plant to maintain stable operation. The requirement is for the cogeneration plant to deliver superheated HP and MP steam to AOG at precisely defined pressure and temperature.
  • Operational independence. In normal operating conditions, the CHP plant will produce enough electrical power to fulfill its own power needs and those of the AOG production complex. Additionally, the CHP plant and AOG production complex should be able to work continuously and independently of the national grid.
  • Facilitation of a planned future expansion of the alumina plant. The planned expansion foresees the addition of a third alumina production line, which would require significant additional quantities of MP steam.

The general consideration that has been followed in the design is that CHP plant operation must not interrupt or hinder production of the production complex in any of its operating modes. HP and MP steam is to be continuously delivered to the alumina plant, except in cases of scheduled shutdowns.

With respect to steam supply, the most serious consideration is that a complete interruption of steam supply that lasts more than 24 hours would leave the raw materials in the alumina process (bauxite and soda) to cool and crystallize, resulting in severe damage and extended plant shutdown. Other parts of the process, such as LP steam consumers, are also defined as critical since they are able to tolerate interruption in steam supply for about 1 hour.

CHP configuration

Extensive design studies and several revisions of the proposed cogeneration scheme established that the most suitable CHP plant would be a 2 x 2 x 1 configuration, as illustrated in Figure 1.


Figure 1. Simplified CHP process scheme
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In view of the possible future expansion of the alumina plant, two distinct design cases have been developed (normal current operation and post-expansion), with each case being subject to performance guarantees. Table 1 defines the main characteristics of the CHP plant under each of the two design cases.

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The required reliability of steam supply comes from the use of a separate dual fuel boiler (DFB) of capacity 110 tonnes/hour of HP steam. If both HRSGs are out of operation at the same time, the DFB becomes the steam source. The DFB can operate on light fuel oil (LFO) as well as natural gas, thus enabling steam production even if the gas supply is interrupted. A permanent stock of LFO will be stored on-site.

Description of CHP plant

The CHP plant comprises two groups of GTs and HRSGs and a single steam turbine. In normal operation, steam from the HRSGs is sent partially to the alumina plant (after attemperation and depressurization) and partially to the steam turbine. The steam turbine has extraction of MP steam. In degraded operating modes, main steam is produced inside a conventional type DFB with either natural gas or LFO.

After steam extraction from the medium pressure stage of the ST, the remaining steam flow is eventually condensed inside a sea-water cooled main condenser which is axial to the steam turbine. Cooling water is taken from the sea and pumped through two 100% circulating system pumps and finally discharged into the sea with a temperature increase limit of 8°C in normal operation. Condensed steam is pumped to plant de-aerators after being pre-heated inside the condensate heater.

Steam sent to the AOG plant is partially recovered as condensate return and sent to plant flash tank de-aerators, where the excess dissolved air is eliminated after flashing inside the flash tanks. The other portion of the return is sent first to the water treatment plant, where it is treated before being re-used in the production of dematerialized water, which is used as make-up water to the cycle. Dematerialized water pumps (3 x 50% capacity) will replenish cycle leakages. Dematerialized water injection occurs at the discharge of the condensate pumps (2 x 100% capacity), and feedwater pumps (4 x 33%) provide water to the HRSG MP and HP water tube bundles.

The operation of the CHP plant is controlled automatically through the use of a Foxboro I/A Series distributed control system (DCS), which is interfaced with subsystem control systems as appropriate. Safe operation of the HRSGs and DFB is provided by use of a burner management system and a boiler protection system .

A further notable feature of the CHP plant is that the gas turbine inlet air is chilled to improve overall plant performance. The chilling units reduce

  • the design ambient temperature of 29°C to 15°C
  • the maximum design ambient temperature of 40°C to 26°C
  • ambient temperature of 15°C to 8.5°C.

A final feature of the plant is the heating of the natural gas to improve efficiency. Increment in natural gas temperature allow savings in natural gas flow for a given gas turbine (or supplementary firing) power output. The gas-receiving station conditions the natural gas temperature. Pre-heating of the natural gas is achieved either by heat exchange with condensate return flow during normal operation or the use of a dedicated auxiliary boiler, which is used in start-up when condensate return is not available.

Electrical section

The electrical installations of the CHP plant are designed principally to enable power export to the network but also to cover the electrical power requirements of the AOG complex if required.

Before the new CHP plant begins operating, the electrical supply to the AOG production complex comes through the existing 150 kV AOG substation with three double power lines, thus maintaining the required degree of availability (n-2 criterion). When the CHP plant begins operating, only two lines will be mandatory, and only required as a back-up source and path for power export.

A 150 kV open-air substation is installed very close to the new CHP units. It comprises three incoming bays, two outgoing bays, a coupler and double busbars. The connection of the existing 150 kV substation with the new 150 kV CHP plant substation is implemented by an overhead HV cable system installed on pillars through the west side of the factory.

To maintain the highest degree of power availability and continuity of power supply, even after a serious incident on the public 150 kV network, the requirement for quick separation from the network by the action of dedicated protection relays (directional overcurrent, df/dt and undervoltage) has been foreseen. The protection philosophy anticipates that the AOG production complex and the CHP plant can be isolated from the public network in the early stage of identification of network failure and that operation shall continue without power interruption, with the following additional considerations:

  • When the disturbance is eliminated and the network is stable, a procedure to resynchronize to the network will be followed.
  • If the separation from the network is not executed properly and the unit protection is activated, the HV unit breaker will open, and the units will turn to ‘house load’ mode, which is defined as covering supply of the auxiliary loads of the CHP plant only. In case the network collapse is persistent, the CHP plant operator is able to initiate the power-up sequence of the AOG factory loads gradually.
  • If for any reason the units are unable to turn in house load mode, a twin pack of black-start diesel engines rated at 3.3 MVA each and connected to the 6 kV switchboard is provided to start up each gas turbine in turn.

CHP plant operational considerations

 

Operational requirements

The CHP modes of operation are defined by three parameters related to the alumina plant operation:

  • HP steam demand
  • MP steam demand
  • the quantity of condensate steam returned from the alumina plant to the CHP plant.

When any of these three parameters change, CHP plant operation will also change. Steam production will be affected, as will make-up water flow. A variation in steam production affects not only the HRSGs’ supplementary firing but also steam turbine operation (and therefore plant power output) and condensate feedwater pump operation.

The alumina plant consists of two production lines, so its main modes of operation are:

  • one line is shut down for maintenance while the other works at 100%
  • one line is in start-up while the other works at 100%
  • the alumina plant is working at 100%.

The CHP plant has been designed to fulfill alumina plant requirements under all the modes of operation, even when conditions are degraded and when working in the most demanding cases (in other words, those that demand less steam).

The CHP plant is designed to be able to continue working, even when capabilities are degraded, to accom-modate the mode of operation of the alumina plant and equipment mainten-ance schedules.

When the alumina plant shifts between different modes of operation, the CHP plant is designed to follow it. The steam turbine does this tracking. Inlet steam flow will vary and increase or reduce output power. Furthermore, if one or both of the GT/HRSG sets is out of operation, the DFB will provide HP and MP steam for the alumina plant and steam flow to the steam turbine will be reduced.

CHP plant operational modes

The CHP plant has been designed to be very flexible, so it can operate in a wide range of modes.

In most of the situations, shifting between different modes and/or sub-operating modes will be the responsibility of control room operators. Additionally, alumina plant control room operators will always have the opportunity to ask cogeneration plant operators for a certain mode of operation (depending on steam/power needs), but they will not have the opportunity to shift between operating modes from the alumina plant control room.

Two main types of operation – open cycle (power generation only) and closed cycle (cogeneration) – are foreseen for the CHP plant.

Open-cycle operation

In open cycle, the gas turbines are the only main equipment in operation. Steam is not produced and electrical power will be the only plant output. HRSG bypass stacks have been incorporated into the design to enable this mode of operation.

Due to the continuous steam requirements of the alumina process and also the reduced plant efficiency in open cycle, it is not expected that the CHP plant will operate for extended periods in open-cycle mode.

Closed cycle – normal operation

During normal operation, the two GT/HRSG sets will operate at 100% load, with supplementary firing, producing HP and MP steam flow. Each one of the gas turbines will be firing 27.98 tonnes/hour of natural gas and producing a steady power output of 124.4 MW.

In normal operation, gas turbine inlet air chillers cool ambient air from the design ambient temperature of 29°C to 15°C. Supplementary firing is applied for a variable amount of natural gas up to a maximum of 2.26 tonnes/hour.

In this mode of operation, the steam turbine operates at a load defined by the available HP steam flow and the MP steam flow remaining after the extraction. The steam turbine maintains HP and MP steam pressures at their rated levels of 65 bar and 15 bar respectively. Steam expanded inside the steam turbine will be condensed and sent back to feedwater tanks, closing the cycle.

This mode of operation allows for the alumina plant to consume a variable flow of desuperheated HP and MP steam. In normal operation, about 85% of the HP-MP steam sent to the alumina plant is recovered as condensate and is continuously pumped from the alumina plant condensate return tanks. This pumped water will be expanded in cogeneration plant flash tanks and partially de-aerated inside de-aerators. It will finally be pumped towards HRSGs. Dematerialized water pumps will be continuously pumping dematerialized water to replenish water losses.

Benefits

The operation of the AOG cogeneration plant will have a significant positive impact on the emissions from power stations and industrial plants in Greece as a whole. Steam requirements for the alumina plant are currently covered by boilers fired by fuel oil, while the electricity required by the aluminium plant is supplied by the Public Power Corporation SA (PPC), the majority of this being from lignite-fired power plants.

Comparing the continuous operation of the CHP plant with a typical 300 MW lignite-fired generation unit, it is estimated that more than 4 million tonnes of lignite will be saved. The corresponding estimated reduction in CO2 emissions is in excess of 3 million tonnes per year.

Beyond the very significant emissions reduction, the operation of the new CHP plant, as a result of its location in the southern part of the national system, will also benefit the overall stability of the national network. The current power generation capacity in Greece is dominated by lignite-fired power plants in the northern part of the country, while the majority of the electricity consumption is in the southern part, especially in the Greater Athens area. The addition of this new CHP plant will therefore contribute to improving the overall balance between the northern and southern parts of the national network.

Conclusions

The AOG cogeneration plant has been designed to satisfy the main design requirements:

  • high reliability and quality of the steam supply to the alumina process plant
  • operational flexibility; enables a wide range of operational modes of AOG’s integrated alumina/aluminium complex
  • operational independence for the AOG production complex, including the capability to work continuously and independently from the national grid if necessary
  • facilitation of a planned future expansion of the alumina plant.

The CHP scheme provides significant net benefits in terms of energy efficiency and reduction in emissions. These are:

  • energy savings as a result of the high efficiency of cogeneration in the new plant compared with the efficiency of current steam production from the conventional boilers fired by fuel oil
  • reduction of emissions of gases such as CO2 and SO2 as the boilers fired by fuel oil will be replaced by high-efficiency cogeneration fired by natural gas.

Paul Smith is Business Development Manager at METKA, Athens, Greece.
e-mail: paul.smith@metka.gr

Co-authors:

Christos Pantzikas, Director of the Engineering Department, METKA SA

Dr Evagelia Bonataki, Head of the Civil Engineering Department, METKA

Iordanis Manousaridis, Director of Electrical Engineering, Rodax SA, Greece

Professor Ioannis Antoniadis, Associate Professor, National Technical University of Athens, Greece

Javier Alonso Zabalo, Director of Thermal & Hydro Projects, Empresarios Agrupados SA, Spain.

METKA SA is an EPC contractor and industrial company, based in Athens, Greece. It focuses on the energy, infrastructure and defence sectors.

This article is on-line: www.cospp.com