The fifth element - Al Taweelah's story3 January 2004
Al Taweelah A1 upgrade came on line and was officially opened during 2003. It was Abu Dhabi's second IWPP and made the Taweelah complex one of the largest power and water production facilities in the world. Now a further development of the site – Abu Dhabi’s fifth privatised IWPP – will create an even larger 4.3 GW complex.
y 1997 Al Taweelah power and desalination complex consisted of two plants, the 240 MWe (net) 29 migd A plant, and the 710 MWe (net) 76 migd B site. In 1998, to help meet spiralling water and power demand locally and in the Emirates generally, the Abu Dhabi government decided to greatly expand the site's capacity, and at the same time to embark on an extensive programme of power and water privatisation. Continuing growth in demand – currently increasing at the rate of 10 % per year – has justified the decision to expand.
Abu Dhabi is the largest and wealthiest of the Emirates and the front runner in creating and privatising power companies. Seven power companies (table below) were created by the unbundling of water and power from the state under Law no 2, March 1998. Liberalisation of the entire sector in legal terms became effective on 31 December 1998.
The first significant step was to be privatisation of the Taweelah A plant, a process that included the first asset sale in the region, at the A1 Extension in mid-2000. At that time it was owned and operated by Abu Dhabi Water and Electricity Authority. The IPP contract involved the purchase and rehabilitation of the existing facility, upgrade of the existing 240 MW, 28 migd plant to a 1350 MW (net), 85 migd plant (the whole becoming the A1 site) by uprating existing plant and adding new plant, and the creation and setting up of a company to own and operate it. This was done by tender, which the Gulf Total Tractebel consortium won in 2000. Meanwhile a fast track project to create a new power station on the site was underway. In March 2000 A2 was completed and commissioned, becoming the first IWPP in the region.
The process of privatisation, or more accurately partial privatisation, has now reached the fifth stage with the offering of tenders for a new extension of Al Taweelah B expected at end-January 2004. Pre-bid meetings with the invited contractors have been provisionally arranged for mid-February.
The structure of Al Taweelah
Broadly speaking the complex consists of three plants, A1, A2 and B. As shown in the diagram (right) A1 and B have both been significantly extended. A1 is owned and operated by Gulf Total Tractebel Power Company. Its sole customer is ADWEA. Its shareholders are ADWEA (60%), Total ((20%)
and Tractebel (20%). Al Taweelah A2, the first independent combined cycle power and desalination plant, was born as part of the general privatisation process, the Emirates CMS Power Company, a joint venture of ADWEA (60%) and CMS Energy (40%) being formed in 1998 to build, own and operate it. Al Taweelah Power Company (ATPC) was fomed in 1993 to build Al Taweelah B in two phases between 1993 and 2000.
Al Taweelah A2 (investment cost AED 2.7 billion) and the A1 upgrade (investment cost AED 5.4 billion) were the first in a series of privatisation projects executed by ADWEA in line with its foreign partnership policy for the sector. The third was the Shuweihat S1 project (owned by ADWEA, International Power and CMS Energy) now close to completion, which will produce 1550 MW of electricity and 100 migd of desalinated water at an investment cost of AED 5.8 billion ($1.1 bn). The Umm Al Nar privatisation announced in April 2003 was the fourth. The fifth project was intended to be Al Mirfa but in September ADWEA, after carrying out a feasibility study into a fifth IWPP plant at Al Taweelah, changed its mind and opted instead for a second extension at Taweelah B, which is currently being prepared for tendering. The projected total investment, that is, the cost of assets and new build, is estimated at AED 7.9 billion. The scheme is likely to involve acquisition of the existing B station (1070 MWe and 100 migd) with the addition of around 1000 MWe and 65 migd. The project will create the largest IWPP in existence.
ADWEA awarded the Umm Al Nar contract to ITM Investment Co, a joint venture of International Power, Tokyo Electric Power Co, and Mitsui. A new project company, Arabian Power Co, owned by ADWEA (60%) and ITM Investment (40%), acquired the existing Umm Al Nar power company assets. The agreement requires the construction of a new 1550 MW power plant and new desalination units of 25 migd capacity. For commercial operation the contracted power capacity of the facility is to rise from 805 MW to 2200 MW by June 2006 and the water capacity to 143 migd. In 2008, the old existing assets will be decommissioned and returned to ADWEA for demolition, leaving the final capacity of the facility at 1550 MWe and 95 migd of desalinated water.
The Taweelah A1 project
The original configuration of A plant consisted of three 85 MW rated Frame 9E gas turbines exhausting to three HRSGs. Together with the output from three 160 t/h, 16 bara auxiliary boilers, their output was fed to four multi-stage flash desalination (MSF) units with a combined output of 29 migd.
MSF is a thermally driven process that involves heating brine with steam and then allowing the hot water to flash off at successively lower pressures. The steam released is condensed as product water and its residual heat recovered by the recirculating cold brine which is then returned to the brine heater. The process is relatively energy intensive but makes maximum use of the heat input. The distillers can use low-grade heat at low steam pressure which has little potential for power generation, raising the overal efficiency compared to a generation-only configuration.
The upgrade was in two parts. The three Frame 9Es were uprated to type PG9171E resulting in a total gross output of 287 MW, and dry low NOx combustors were fitted. At the same time the desalination units were refitted to uprate them to 8 migd.
The power section consists of five GE 9171E gas turbines, each having a site rating of 107 MW. Each GT has an associated HRSG fitted with supplementary firing to increase the HRSG output as required. Steam from both the new and upgraded HRSGs passes to three backpressure steam turbines each producing 203 MW. Residual low pressure steam is sent to the desalination plant which consists of fourteen Sidem 3.8 migd MED units.
The existing shared facilities consisted of a seawater intake feeding a channel with two branches – one for the existing plant and A2 , the other for the B1 and B2 plants; brine outfall channels and surface culverts; and medical and fire facilities. Also a mosque, with Imam's quarters – under Abu Dhabi law a mosque must be included in all developments where the workforce is large.
The plant extension required additional shared facilities, including rehabilitation of the emergency generators, two 100 % capacity fuel gas systems for the GTs and two for duct firing in the HRSGs, modification of the demineralisation plant to provide a 12 hour reserve, new cooling water systems, four potable water storage tanks each of 10 mig net capacity, a new fire protection system, and a process waste water facility and neutralisation basin.
The plant was constructed in two phases between 1993 and 2000. Phase 1 (units 1-6) consisted of six steam raising boilers driving the turbine/generators and the distillation units, and desalination units producing 6x122 MWe maximum with full simultaneous water production, which is 76 migd. Construction started in 1993, the first power and water being exported in the summer of 1995. The plant is fuelled by piped-in natural gas with crude oil supplied by tanker as the back up.
Phase 2 (Units 7 to 9) aka the B extension, or B2, was built on a compact site immediately to the east of the phase 1 plant and consists of a 2 + 1 gas/steam turbine combination driving the generators, with desalination distillers utilising steam from two HRSGs. It was built, and commissioned between 1999 and 2001, by a consortium made up of Eastern Bechtel Co and Hitachi Zosen.
Phase 1 configuration
With a steam generator as prime mover, the most economical means of power and water production is to feed its output both to the turbines and distillers. All six steam turbines can be operated in extraction or condensing mode. The air-cooled generators are driven in a single shaft configuration.
The six natural circulation boilers, each with 12 burners, can fire natural gas or crude oil or a mixture of both. Each boiler includes economiser, evaporator, superheater and a local control station.
The plant is operated on a dual-fuel system. Natural gas piped in at a pressure between 25 and 42 bar, which is further reduced for the boilers. Gas consumption per boiler is 40 m3/h at full power and water production. 3 x 11kV diesel generators rated at 3030 kVA are installed at site for back up to shutdown the power station safely in case of emergency.
The desalination plant, designed and built by Fisia Italimpianti of Italy, includes a large offshore seawater intake system and an onshore seawater intake and pumping station. There are four storage tanks with a capacity of 86 m3 each. The six 20-stage MSF distillers can accept steam from the turbine exhaust or directly from the boiler. It takes about 8 litres of seawater to produce 1 litre of potable water.
The offshore channel is approximately 2 km long and was designed with sufficient capacity for the B site and the (then) future A2 plant.
Phase 2 configuration
The plant installed in the extension project can produce 8.2 million kWh/day of electrical power and 23.1 migd of potable water. It is controlled from new panels installed in the existing central control room, but may also be controlled locally.
The fast-track contract commenced in February 1998. First export of power, with the gas turbine operating in simple cycle mode, followed in June 1999 and first potable water in May 2000.
The power plant comprises two gas turbines rated at 97 MW each and one back-pressure steam turbine rated at 143 MW. The two gas turbines can be operated in simple-cycle or combined-cycle mode. According to demand and servicing needs, the HRSGs can drive the steam turbine directly, which in turn exhausts low pressure steam to the 3x7.7 migd MSF distillers or sends high pressure steam via a pressure reducing station directly to the distillers if the steam turbine is out of service. In these circumstances the HRSG can produce sufficient steam from the gas turbine exhaust for full water production. Primary fuel is natural gas, with diesel oil (gas oil) available for emergency back-up.
The overall cost was reduced because many of the auxiliaries required for power and steam generation already existed in spare capacity built into phase 1. However, a new high pressure gas reducing station was constructed next to the site and the original seawater intake was extended by the addition of a pumping station of 72 m3/h capacity. Scope of the works also included an on-site mosque.
New 132 kV switchgear based on gas insulated (SF6) designs was added to the existing 132 kV and 400 kV substations. 3 x 25 MVA auto-transformers act as interbus transformers between the 132 kV (units 1,2,7,8,9) and 400 kV (units 3 to 6) system.
The site includes a ‘state of the art’ laboratory for ongoing maintenance analysis as well as research into metallurgy, corrosion analysis, gas quality control and quality of potable water.
This project, which would create Abu Dhabi’s first integrated water and power producer, involved the design, construction and operation of 710 MW (net) of power together with 50 migd of fresh water capacity on a BOO basis. The contract included extending the existing 400 kV substation at the B plant and the construction of a new potable water pumping station and open discharge channel to the existing facility serving the A1 and B plants. The power island consists of three 185 MW gas turbines supplying steam via HRSGs to two back pressure steam turbines rated at 113 MW.
In this installation exhaust emissions are controlled by Siemens’ dry low NOx hybrid burner ring, and when burning oil, by water injection. The desalination island is based on four MSF distillation units each of 12.65 migd capacity fired by exhaust from the steam turbines. The product is remineralised to render it drinkable and pumped to storage tanks with a capacity equal to a day’s output. The development included an extension to the existing 400 kV grid station at the B plant along with the construction of a new potable water pumping station and a new open discharge channel, but the project benefited from existing facilities developed for the A1 and B plants.
The power island comprises three industrial gas turbines of a type (V94.3A) that has now completed well over a million operating hours at Didcot B, Genelba, Seabank, and Tapada do Outeiro power stations. Each is rated at 185 MW at 46°C site conditions and has an associated single pressure HRSG. Two back pressure steam turbines rated at 113 MW generate power from the main steam flow. Supplementary firing is used to enhance and provide flexibility in steam production. Export from the power island is by 400 kV cable to a substation on the 400 kV transmission system.
Exhaust heat recovery
Each HRSG is a horizontal gas flow, natural circulation, single pressure design manufactured by Doosan (formerly Hanjung) but based on a well established ABB licensed design. Either gas or distillate fuel oil can be burned in the HRSG to enable the boiler evaporation to be adjusted to meet desalination steam demand irrespective of GT power.
The steam turbines are double axial flow machines with the main steam inlet at the centre of the cylinder. The system includes two 100% steam bypasses (to the steam turbines) to permit water production to continue during turbine maintenance or when minimum power generation is required. Two sea water cooled dump condensers are provided with sufficient capacity to condense the steam flow to one desalination unit, permitting electrical generation to continue at full output during maintenance of one desalination unit.
Extensions to the existing 400 kV Taweelah B GIS were to provide for the connection of four additional circuits from the generator transformers to the Transco 400 kV transmission system, although export of power from the site is via existing 400kV transmission connections which have adequate margins for the additional generation capacity even under single circuit outage conditions.
New pumps rated at 30 migd each for delivery of water to the high pressure water header and transmission mains were installed, along with a monitoring system to enable sizing the surge vessels necessary to avoid water hammer effects. The pumps themselves are horizontal units with hydraulic variable speed couplings to permit regulation of the discharge flow and pressure. A new shared facility comprised a length of open channel for discharge of cooling water from the new plant to the existing discharge channel shared by the A1 and B plants.
Al Taweelah plant details
GTs: 3 x GE frame 9E at 85MW
3 HRSGs, plus 3 aux boilers
Water: 4 Sidem MSF; 3 x 7.2 migd, 1 x 7.6 migd
Gross outputs: 255 MWe and 29.2 migd fresh water
Boilers: 6 Deutsche Babcock at 650 tonne/h steam.
STs: 6x1 ABB 122MW
Water: 6x12.8 migd Fisia Italimpianti MSF desal units.
Gross outputs: 732 MWe, 76.8 migd water
GTs: 2x97 MW GE with Aalborg HRSGs
ST: 1 x GE 143 MW
Water: 3 x Hitachi-Zosen multistage MSF at 7.7 migd
Gross outputs: 342 MWe, 22.1 migd water
GTs: 3 x Siemens V94.3A at 185MW
3 HRSGs with supplementary firing
STs: 2 x Siemens NG90/90 113 MW back pressure.
Water: 4 x Hanjung (now Doosan) MSF at 12.65 migd
Gross outputs: 780 MWe, 51 migd water
A1 upgrade and extension
Upgrade of existing 3 x GE frame 9E to 3 x PG9171E at
95.5 MW, fit dry low NOx combustors
Upgrade of existing MSF units to 8 migd each
A1 plant extension
Five GE 9171E GTs at 107 MW each with HRSG with supplementary (duct) firing
STs: 3 ABB/Alstom at 203 MW
14 Sidem MED desalination units at 3.8 migd.
Gross outputs: 1144 MWe, 59 migd water
TablesStructure of ADWEA following liberalisation Al Taweelah development - timeline