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Cogeneration plant to boost output at Dubai Aluminium

The Dubai Aluminium Company’s huge electricity and steam demands are supported by a captive power station which includes three combined cycle power plants. The company is now working with Alstom to build a cogeneration power plant to guarantee uninterrupted steam and power production at the site. Pierre-jean Matherat reports.

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The new unit within the wider Dubal complex

Dubai Aluminium Company Limited (Dubal) is the leading supplier of aluminium in the United Arab Emirates. As a highly electricity-intensive process the company operates its own power plants to provide electricity for the smelting process and steam for use in a desalination plant.

In 2006, with the addition of another aluminium smelting potline, the company expanded its generating capacity with the award of a contract to Alstom for the CCPP22 combined cycle plant. This added 430 MW of new capacity to the 1750 MW produced by the Kestrel and Condor 1 and 2 power plants.

Now in an effort to guarantee full steam output and boost the overall output of the three older combined cycle units, Dubal is working with Alstom to develop a new cogeneration plant that is integrated into the existing plant in a novel arrangement.

 

 

POWER PRODUCTION AT DUBAL

 

October 2009 marked the completion of Dubal’s 30th year of operation. From a relatively small smelter operation utilising three potlines which produced 136,000 tonnes of aluminium per annum in 1979, Dubal has expanded its operations to encompass nine potlines with the capacity to produce more than 960,000 tonnes of quality hot metal aluminium each year for clients in more than 48 different countries.

To ensure a reliable supply of electricity for producing aluminium, Dubal has a dedicated power plant on-site. The hot gases from the gas turbines in the plant are used to produce steam, which in turn provides the energy to produce sweet water via thermal desalination of seawater. By utilising the residual energy from the power generation operation to produce water, the overall energy efficiency of Dubal is enhanced.

Dubal produces high quality potable and distilled water by distillation of seawater through six multistage flash (MSF) evaporators and has a total water production capacity of 30 million imperial gallons per day (MIGD).

The Dubal power production units essentially offer an island production site. It is a site at which Alstom has had a long involvement.

Just over 10 years ago, Alstom provided Dubal with the first ‘add-on’ for its Condor power plant. Under this contract, Alstom converted two GE Frame 9E turbines in open cycle to a combined cycle plant. Alstom provided and installed a steam turbine and heat recovery steam generator (HRSG) for the project. Then in 2003, Alstom won another add-on contract, this time for the Kestrel power plant, to convert another open cycle Frame 9E gas turbine to a combined cycle plant.

In these add-on projects, Alstom added to the existing gas turbines: heat recovery steam generators, a steam turbine and its generator, the condenser, cooling system and the water/steam loop equipment (piping feedwater pumping etc.)

The Condor and Kestrel projects have not only served to strengthen Dubal’s position as an important player in the international aluminium market, but have also increased the company’s importance as a key contributor of electrical power production to the national grid.

Building on the strength of these add-on projects, Alstom was then awarded a €200 million ($272 million) turnkey contract for its first complete project for Dubal. The CCPP22 plant comprises a KA13E2 combined-cycle power plant containing two Alstom GT13E2 gas turbines, two heat recovery steam generators, one steam turbine and three generators. This plant entered commercial operation at the end of 2007.

 

A NEW CONCEPT

 

With the new plant in operation, Dubal’s next immediate task is a planned scheduled maintenance of the gas turbines at the Kestrel, Condor 1 and Condor 2.

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The new GTX cogeneration unit

As the work on CCPP22 neared completion, Dubal approached Alstom to investigate if there was a way of replacing the steam generated by the HRSGs of Kestrel and Condor 1 and 2 when any GT was shutdown. The company came up with the idea of introducing a boiler called ‘boiler X’ that would be able to replace any of the six HRSGs of Kestrel, Condor 1 and Condor 2. This was start of the development of a technically challenging cogeneration plant originally called GTX HRSGX, and then GTX.

Dubal made a full assessment of such a plant, considering the operation of the entire site and keeping in mind the maintenance work that could take place every six months on one of the six gas turbines at the site.

While such a project was economically feasible, Alstom found that there could be additional value if the GTX plant could also be used to feed steam to the combined cycle plants in normal operation to boost the output of the steam turbines. This would essentially give GTX a dual purpose, i.e. to replace HRSGs of GTs that are shutdown or to generate extra steam for more electricity production from the existing combined cycle plants.

It was a solution that led Dubal to award Alstom an approximately €123 million contract in June 2007 for a 150 MW cogeneration power plant. Under the full turnkey EPC contract, Alstom is building a plant that will include one GT13E2 gas turbine, an air-cooled TOPAIR turbo-generator, a new heat recovery steam generator and an advanced ALSPA distributed control system (DCS).

The new concept of GTX project involves three old combined cycle generating plants (generally, a ‘unit’ involves just one GT) plus the new cogeneration plant operating under one single integrated control system.

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Construction of the GTX unit in the Dubal complex

The GTX plant has a two-pressure heat recovery steam generator (HRSG) that recovers exhaust heat from the gas turbine to generate high pressure (HP) steam at 81 bars and 510°C. The HRSG can supply 220 tonnes/hour of HP steam; more than enough to fully replace the steam of one of the six existing HRSGs. GTX also produces electricity directly for Dubal’s own consumption.

The new scheme has several modes of operation in terms of steam use and delivery.

 

Steam export mode

 

In the steam export mode, steam is exported to Dubal’s seawater desalination plant to increase the production of drinking water. Here, steam is fed at 20 bar to the desalination plant.

 

Replacement mode

 

The second steam replacement mode is employed when a gas turbine is shut down for maintenance. Here, the steam capacity of the HRSG associated with the gas turbine in the combined cycle unit is replaced with steam from the GTX HRSG. This means that if one of the HRSGs is shut down, the associated steam turbine sees no difference and can continue operating at full load.

 

Overall plant boosting mode

 

When all units are running normally, steam from GTX is distributed between Kestrel, Condor 1 and Condor 2. This is the most complex mode of operation. The existing combined cycle units are based on Frame 9E gas turbines operating at a load whereby the steam turbines still have enough capacity to receive additional steam from GTX. This additional steam will allow the steam turbines to operate at their maximum load, allowing the steam turbine in each combined cycle unit to deliver an additional 15 MW. This will increase the overall efficiency of Dubal’s power production process. In this mode, steam can still be delivered to the desalination plant.

At the same time as feeding the steam to the three steam turbines of the power plants, steam is exported to the desalination plant – maintaining the pressure in the large header for the steam distribution. The regulation of the plant is adjusted to feed steam to the desalination plant so that there is no wastage of steam.

 

TECHNICAL CHALLENGES

 

Developing such a complex plant was a challenge, presenting several key difficulties; in fact there were several small ‘projects’ within the project.

With a number of technical issues needing to be clarified and solved, Alstom began work immediately after receiving the notice to proceed in June 2007. The pre-assessment of the feasibility of the concept, which was handled during the tendering stage, took 5-6 months. This compares to typically 8-12 weeks for a standard cogeneration plant.

The huge steam distribution network that is supported by the 1 km long pipe rack was difficult to implement and required a huge engineering investment.

Each of the six HRSGs has HP and LP connections that will receive steam from the GTX plant. With GTX located between Kestrel and Condor 2 – about 1 km from each – Alstom had to build a huge pipe rack, made of steel and concrete, to support the steam distribution system.

Building such an extensive pipe rack was made more difficult by the site constraints. There is a large underground network of cables and piping at the site, which would have caused unforeseen obstructions. This led to a complete review and study of the pipe rack prior to starting work at the site.

At a greenfield site, there are typically three or four types of foundation that can be used for any part of the pipe rack. The integration of GTX, however, called for about 70 different foundations and 69 different designs. This means that there are only two similar foundations along the entire pipe rack.

The other major challenge was the control of the overall scheme in the ‘plant-boosting mode’. Here, the challenge is ensuring the full balance of steam and water between the combined cycle units.

This complex concept required a full upgrade of the existing DCS so that GTX and the entire plant, including Kestrel and Condor 1 and 2, can be fully operated from a single control room.

In implementing the new control system, Alstom had to ensure the various elements of the existing systems could communicate with each other with the proper information exchange. The Condor units use ALSPA version 4, while Kestrel uses ALSPA version 5; Alstom’s latest ALSPA version 6 is being installed for the control of GTX plus the combined cycle units.

The new control room allows Dubal to control power export and import between its power production site and the Dubai Electricity and Water Authority (DEWA), as well as the steam distribution and water production. The entire plant runs automatically, with just five or six operators in the control room.

 

PROJECT SCHEDULE

 

Despite the challenges, the team at Dubal has made progress. First ignition of the gas turbine for GTX was in October 2008, with gas turbine takeover in March 2009 and first synchronization in October 2009.

A major milestone was reached in August 2009 with the first steam and the start of the first steam blows through the more than 1 km pipe work to the HRSGs. This represented the first live test condition of the plant.

The final part of the cleaning, the bypass operation, was completed in October. During this operation, the GTX HRSG feeds each of the combined cycle units up to the bypass valve to ensure the final complete cleaning of the steam feed line. The new GTX plant began hot commissioning in October 2009.

Despite its complexity, the construction has been a great success. Pipe rack installation can be difficult for site workers. But notably, since the beginning of construction there have been zero injuries. This has been achieved through strict adherence to safety rules.

This project is a good demonstration of Alstom’s competence in providing tailor-made solutions: Alstom has a long experience from many dedicated power plants for Aluminium smelters and desalination plants. Also Alstom’s ‘Plant Integrator’ approach and turnkey EPC capability were important for the successful completion of this project.

The relationship with Dubal was important in the development of the plant, demonstrating the need for a strong partnership between the contractor and owner in such a complex project.

Pierre-jean Matherat is project manager with Alstom Power, Belfort, France.

Email: pierre-jean.matherat@power.alstom.com

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