DUBAL power expansion project honoured in US

The innovative thinking and quest for operating efficiency through continuous improvement at Dubai Aluminium (DUBAL) has again been recognised on the international stage. DUBAL’s unique GTX project, which was presented by Tayeb M M H Al Awadhi (Vice President: Power & Desalination) at Power-Gen Asia, held in Singapore during 2010, has won the Project of the Year 2011 “Honourable Mention Best Gas-Fired Project” at the Power-Gen International 2011 conference, held recently in Las Vegas, USA.

To appreciate the significance of the award requires an understanding of the GTX project, its origins and the role it plays. The project’s setting is as important. In this regard, DUBAL is one of the largest single-site aluminium smelters in the world (producing more than 1 million metric tonnes of hot metal per annum). The smelter complex includes a 2,350 megawatt (MW) power station (at 30˚C), a water desalination plant and a large carbon plant. The power station is designed to provide both electricity to the smelter reduction process and saturated steam to the water desalination plant. The present average aluminium smelting power demand is about 1,900 MW, amply covered by the generation capacity of the power plant.

Back in 2006 (i.e. prior to the GTX project), the DUBAL power plant comprised 22 Gas Turbines and 7 Steam Turbines. The average generation capacity of 2,115 MW met the 1,750 MW average demand of the site at that stage. The plant configuration fulfilled the continuous load supply requirement with sufficient spare generation capacity to cover the loss of the highest generating unit while one large unit was on planned maintenance outage. The annual average station efficiency was 43 per cent, with three to four Frame 9B Gas Turbines being required to meet the plant’s power and water demand.

An analysis of the planned extensions of DUBAL’s existing potlines indicated that the electricity generation requirements within the DUBAL complex would rise to about 1,850 MW by 2010. A feasibility study, conducted in 2006, showed that DUBAL’s reserve capacity would not be sufficient to cover the additional power demand. Furthermore, the complex would have to rely on less-efficient machines, which had already partially completed their asset lifetime. It was clear that an increase in the installed generation capacity of the DUBAL Power Plant complex was needed.

The same feasibility study showed that the annual average generation efficiency of the DUBAL Power Plant complex could be improved by installing a GTX Cogeneration Plant; and that the annual fuel consumption for power generation purposes could be reduced by about 4 per cent. The expected partial operation of the proposed GTX Cogeneration Plant on distillate oil due to possible gas shortages in summer further improved the profitability of the project.

Given these benefits, the project was given the go-ahead – with the primary objective of implementing a GTX Cogeneration Plant being to improve the power generation efficiency and decrease the environmental impact of the DUBAL Power Plant complex. This would be achieved by integrating the proposed GTX Cogeneration Plant into the existing DUBAL Power Plant infrastructure, thus enabling better fuel utilisation by increasing the efficiency of electricity generation and water production within the complex; and minimising industrial gas emissions, specifically oxides of nitrogen (NOx) and carbon dioxide (CO₂). A secondary objective was to increase the total power generation capacity of the DUBAL Power Plant by leveraging the ability of a GTX Cogeneration Plant to raise the installed generation capacity.

Unlike a conventional Combined Cycle Power Plant (CCCP), DUBAL’s GTX project entailed installing one large state–of-the-art Gas Turbine and one dual pressure Heat Recovery Steam Generator; and then supplying the steam heat energy to three existing Frame 9E Steam Turbines and the water desalination plant. The GTX Co-generation Cycle is designed to work in three modes:

Additional steam heat energy to operate the existing Steam Turbines on peak load;

Additional steam heat energy to replace one existing Heat Recovery Steam Generator; or

Steam heat energy to the water desalination plant.

In essence, the new technology and new process concept is not only capable of accommodating the increase in power production and steam management, but is also more flexible to accommodate the downtime and maintenance requirements resulting from operating the existing Steam Turbines on peak load.

By enhancing the Steam Turbine output to peak operation, or replacing an existing Steam Generator while on maintenance, the GTX plant has enabled an annual average increase in overall fuel free Steam Turbine output of over 50 MW while the surplus steam directed to desalination plant produces additional water. Accordingly, the annual average station efficiency rose from 43 per cent to 44.5 per cent.

Moreover, the new GTX block has introduced 220 MW of reserve capacity, which has been utilized to increase the load carrying demand by 150 MW while the spare 70 MW has reduced demand for one Frame 9B Gas Turbine. Switching-off one Frame 9B unit has resulted in gas savings of about 15 mscfd, and a concomitant reduction in CO₂ emissions of 300,000 tonnes per annum.

The GTX project, which cost US$183 million, is the first of its kind in scale and application, as integration with CCPP and desalination has never been done before. It’s also a first in the aluminium industry, where no similar GTX projects are known.