Fujairah 2: Power and water for Abu Dhabi

Fujairah 2 was commissioned in 2010 to help meet Abu Dhabi’s growing demand for water and electricity Source: Alstom
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Fujairah 2, Abu Dhabi Water and Electricity Authority’s largest independent water and power project, was inaugurated on 9 May. This project is also a milestone for OEM Alstom, as the first project in the Middle East to utilize its GT26 turbine, as well as the company’s first IWPP contract in the region.

MEE Report

Fujairah 2, a combined-cycle power and desalination plant in the United Arab Emirates (UAE), is the seventh and largest independent water and power project (IWPP) to be commissioned by the Abu Dhabi Water and Electricity Authority (ADWEA) since the emirate reorganized its power and water sectors in 1998.

The plant is located in the emirate of Fujairah, the most easterly of the emirates in the UAE, adjacent to the site of an earlier power and water plant. The new IWPP, commissioned in 2010, has a generating capacity of 2000 MW of electricity and 130 MIGD (million imperial gallons/day, equivalent to approximately 600 000 m3/day) of potable water.

The deal to build the plant followed a competitive tendering process under which bids were solicited by ADWEA in April 2007. The winning consortium sealed its contract by signing a power and water purchase agreement in August 2007, following which the engineering, procurement and constructio (EPC) contract was signed between the developers and the Alstom-Sidem consortium.

A consortium comprising ADWEA, Marubeni Corporation and International Power created a special purpose company, Fujairah Asia Power Company (FAPCO), to own the plant. This company awarded the EPC contract for Fujairah 2 to the consortium of Alstom and Sidem.

According to Marubeni, the total cost of the project was $2.8 billion. Alstom’s share of the contract was worth $1.6 billion, equivalent to a unit cost of $730/kW for the power generation part of the project. Sidem’s share of the EPC contract was estimated to be worth a further $800 million.

Project Design

For drinkable water the UAE has a large demand that does not vary substantially with the seasons. In contrast, peak power demand depends on the season. Demand for power is high during the summer, due to increased air-conditioning loads, but falls during the winter months as local temperatures fall. As a result, the new plant at Fujairah must be able to supply a relatively constant output of potable water while its power output varies from a maximum of 2000 MW net power to about 900 MW during the winter months.

To optimize the plant’s performance, EPC contractor Alstom took advantage of what it calls its ‘Plant Integrator’ approach, developed for designing optimal, tailor-made power plants. With gas turbines, steam turbines and heat recovery steam generators (HRSG) for the plant all manufactured in-house, the company was in a position to choose the best combination of components to suit the demands of FAPCO.

Flexibility in the plant design is achieved in several ways. Basic gas turbine combined-cycle plant performance can be adjusted by using gas turbine inlet air cooling, a strategy that increases efficiency when the ambient temperature is high as in the Gulf. Alstom uses evaporative cooling systems, bringing humidity close to saturation, as well as additional high fogging systems for an optimized, EOH-neutral power boost

Various HRSG options are available including drum or once-through designs; in the case of a desalination plant a duct-fired boiler may prove the optimum choice. Steam turbine and condenser design can also be chosen to give optimum performance under specified conditions. Variable steam extraction from condensing-extraction steam turbines provides additional flexibility between power and water production.

In the case of Fujairah, the overall design also took into consideration the desalination part of the project. At Fujairah, the desalination plant is made up of two sections: one based on multiple effect distillation (MED) and the other on reverse osmosis (RO). The largest of these, the MED section, takes steam from the combined-cycle plant to generate potable water.

The smaller section based on RO is driven not by steam but by power. This combination allows for the optimization of steam and power output from the combined-cycle plant in order to provide constant water production as power demand varies with the seasons. Optimization is a cyclical process in which each optimized design is further refined until the solution that best matches the customer’s demands, both from a cost and performance perspective, is achieved.

In the specific case of an IWPP plant such as Fujairah 2, Alstom’s iterative methodology includes optimization based on the following parameters: baseline desalination plant performance; a balance of condensing steam turbines with extraction and back pressure steam turbines; selection of pressure level for steam extraction from steam turbine for use in the desalination plant; operational pressures within the HRSG; proportion of supplementary firing within the heat recovery steam generator; ratio of water production from MED versus RO; and the yearly water/power demand profile.

Fujairah 2 Layout

In order to provide both the power output and the flexibility required by ADWEA, the design chosen for the Fujairah 2 power and desalination plant involved three gas turbine combined-cycle blocks incorporating five gas turbines and three steam turbines, a 100 MIGD MED desalination plant based on twelve 8.3 MIGD MED units and a 30 MIGD RO desalination section. This configuration was found to offer the optimum in terms of both flexibility and economy.

The Fujairah 2 plant while under construction on the UAE’s east coast Source: Alstom

The three combined-cycle blocks are not identical. Two are multi-shaft blocks based on Alstom’s KA26-2 combined-cycle units while the third is a multi-shaft KA26-1 block. The multi-shaft KA26-1 comprises a GT26, 50Hz gas turbine that is connected directly to a Topgas hydrogen-cooled generator. A single case STF15C back pressure steam turbine is connected to another generator

The other two combined-cycle blocks are based on KA26-2 multi-shaft configurations. In this case, each block has two GT26 gas turbines, each mounted on a single shaft with a TOPAIR air-cooled generator. The exhaust from these pairs of gas turbines is used to raise steam to drive a single, three-case STF30C condensing steam turbine, which has its own Topgas hydrogen-cooled generator.

All the gas turbines are provided with inlet air-cooling to raise efficiency under the high ambient temperatures frequent at Fujairah. Each gas turbine also has its own HRSG, which is fitted with duct firing to improve flexibility by allowing additional steam to be raised within the steam generator when required.

The 12 MED units of the first section of the desalination plant are driven using steam extracted from the two condensing steam turbines and from the exhaust of the back pressure steam turbine. This provides maximum output from the MED units during the summer. But during the winter when power demand is lower, overall steam output will be reduced as well. This can be compensated for, in part, by increasing the duct firing within the HRSGs.

The second section of the desalination plant is based on RO. This is driven by power rather than steam and so can be operated independently of steam output. The RO section can therefore be used during the winter to maintain water output when power demand is low but its use can be reduced during the summer when the combined-cycle units are operating at maximum power.

The control system at Fujairah 2 is based on Alstom’s ALSPA P320 distributed control system. This system will be used to control both the power plant and the desalination plant. Control is divided into three sections: one for the power plant, one for the MED plant and one for the RO.

Grid connection is via a gas-insulated switchgear substation already at the site. This was installed by the Abu Dhabi grid company, Transco, a wholly owned subsidiary of ADWEA, to provide connection for the Fujairah 1 power plant at the site. This substation links into a 400 kV network that is part of the Abu Dhabi grid system. But an additional 400 kV link is being built between Fujairah and Sweihan in Abu Dhabi, via Ras Al Khaimah and Sharjah, to carry additional power from the new plant.

Gas Turbine

The prime movers for the three combined-cycle blocks at Fujairah are five of Alstom’s GT26 gas turbines. At Fujairah 2, these will be capable of dual-fuel operation, burning natural gas under normal circumstances, with the capability of operating on liquid fuel if the gas supply is interrupted.

The water inlet for the desalination unit of Fujairah 2 Source: Alstom

The GT26, and its 60 Hz variant the GT24, are the latest of Alstom’s large power plant turbines. Today, the GT26 has a rated output in open cycle of 296 MW and an efficiency of 39.6 per cent. The unit, however, is designed for combined-cycle use and under these conditions when closely coupled with a steam turbine it can achieve efficiencies approaching 60 per cent.

The design of the GT26 incorporates a 22-stage subsonic compressor that takes inlet air and compresses it before introducing it into the unit’s combustor. In the case of the Fujairah plant, the inlet air is first cooled in order to gain operational efficiency before entering the compressor. The combustion system in the GT26 is divided into two, sequential stages, a strategy which Alstom claims allows it to achieve the highest operational flexibility and to maintain low NOx emissions over a very wide operation range.

The first combustion stage is an annular combustor with 24 retractable burners (this is designated the EV combustor by Alstom). Fuel and compressor air are premixed before entering these combustors. The hot exhaust gases then pass through a one-stage, high-pressure turbine section before entering the second (Sequential EV or SEV) combustor.

As with the first, this is an annular combustor with 24 burners in which the injected fuel ignites spontaneously when it mixes with the hot exhaust gases from the first combustor. Exhaust from the second combustor then enters the four-stage low-pressure turbine section.

Alstom’s data show that sequential combustion allows the temperature of gases exiting the first combustor to remain constant from between 100 per cent and 30 per cent load, while the temperature of the gases exiting the second combustor starts to fall once load drops below 40 per cent. Gas turbine exhaust gas temperature is therefore maintained at a roughly constant level between 100 per cent and 40 per cent load, and this results in good part load combined-cycle performance.

The constant combustion and exhaust gas temperature profiles over a wide range of gas turbine operating conditions also makes it easier to maintain low NOx emissions. Measurements on many GT26 units showed that these were well below 25 vppm between 100 per cent and 40 per cent load.

HRSGs, steam turbines and generators

The five gas turbines at the Fujairah 2 will each be equipped with their own HRSG. All five boilers are identical and are supplied by Alstom. Each is fitted with duct firing, which will be used to supplement the steam output when electrical load is reduced during the summer in order to supply sufficient energy to drive the MED desalination units.

Two different steam turbines will be utilized at Fujairah 2. The two KA26-2 multi-axis combined-cycle blocks will be equipped with STF30C three case floor mounted condensing steam turbines. The multi-shaft KA26-1, in contrast, will be fitted with a STF15C single case back pressure steam turbine, again floor mounted. Both are designed specifically for combined-cycle operation.

The STF15C in the multi-shaft configuration provides up to about 136 MW of power at Fujairah 2. The back pressure steam turbine provides exhaust steam for the MED desalination units. Meanwhile, the two STF30C condensing steam turbines each provide up to 275 MW depending on the unit load. These two turbines are provided with steam extraction points from which steam can be taken to drive the MED desalination units. As with all the company’s steam turbines, the units for Fujairah have welded rotors, separate high-pressure and intermediate-pressure casings, and single bearing supports between turbine casings.

As with the steam turbines, two types of generator are also being used at Fujairah 2. The five gas turbines and the back-pressure steam turbine are each equipped with one of Alstom’s TOPAIR generators, which is directly coupled to the gas turbine (or steam turbine) on the same shaft. The TOPAIR generator is a totally enclosed, water-to-air cooled generator. In this design, stator windings are indirectly cooled while the stator core and the rotor are radially cooled.

The other generators at Fujairah are based on Alstom’s Topgas hydrogen-cooled design. This uses a hydrogen-to-water cooling system for higher efficiency, enabling larger power handling. As with the TOPAIR units, stator windings are indirectly cooled while the stator core and rotor are cooled directly.

Desalination

Desalination at Fujairah 2 is performed using two different technologies, MED and RO. The first is a thermally driven distillation process, which utilizes steam from the combined-cycle plant while the second is electrically driven. The combination of the two technologies provides the most economical solution for enabling potable water output to be maintained as the power output varies with the seasons.

A computer-generated image illustrating the principal elements of Fujairah 2’s layout Source: Alstom

MED is carried out in a cascaded series of cells called ‘effects’. Each cell contains horizontal bundles of tubes into which steam is introduced while cold seawater is sprayed over the tubes from the top of the cell. Steam from the combined-cycle plant is fed into these horizontal tube bundles in the first cell.

This steam causes evaporation of water from the seawater and at the same time the steam is condensed within the tubes in the cell to produce pure distillate. The water produced from the condensation of steam within the tube bundles is collected, as is the brine that emerges from the bottom of the cell.

Meanwhile the steam generated within this first cell is fed into horizontal tube bundles in a second cell where the whole process is repeated, but with slightly cooler steam and with the pressure within the cell maintained at a lower level than in the first. By repeating this in a series of cells, or ‘effects’, most of the heat energy contained in the steam from the power plant can be exploited to evaporate water from seawater to provide distillate, leading to a highly efficient process. Steam from the last cell in the series is condensed using a seawater-cooled condenser.

When steam is not available, the MED units can still be operated using an electrically driven mechanical vapour compressor. This reduces the output of each MED unit. Fujairah 2 will be equipped with 12 MED units each capable of producing 8.3 MIGD of potable water when operating on steam from the combined-cycle plant. To enhance the flexibility of the desalination operation further and allow optimum output when power demand is low, Fujairah 2 is also equipped with a
30 MIGD RO unit.

Since RO requires power it costs more to operate than MED. However, it offers a good method of supplementing water output in the winter when power demand is low because power generated from the combined-cycle plant can be diverted from the grid to the RO unit.

Allied to the IWPP project itself are three transmission schemes. The first involved the construction of a new 240 km gas pipeline from Taweelah in Abu Dhabi to Fujairah to supply the plant with natural gas. A new water pipeline has also been built to link Fujairah and Abu Dhabi. In addition a 400 kV transmission line has been constructed. As a consequence, Fujairah 2, as well as increasing both power generating and water production capacities in the UAE, has extended the region’s gas, power and water network infrastructures.


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