Dubai Electricity & Water Authority (DEWA) is a utility responsible for power generation, desalinated water production, transmission and distribution of electricity and water in the Dubai Emirate. DEWA owns and operates a large power and desalination station complex at Jebel Ali, Dubai, consisting of Stations ‘D’, ‘E’ & ‘G’ and a peak lopping station ‘H’ at Aweer. All these power stations operate on natural gas as primary fuel and medium fuel oil/diesel oil as secondary fuel. The present total installed capacity of DEWA is around 2900 MW of power and 580 litres/day (128 million imperial gallons/day, MIGD) of desalinated water.
In November 1999 DEWA awarded a turnkey project contract to repower/convert the existing Jebel Ali conventional thermal power and desalination Plant ‘D’ Phase II to combined cycle/cogeneration operation.
The scope of work under the repowering project covers: the addition of three gas turbines associated with three waste heat recovery boilers; construction of a new 400 kV substation ‘D’; interconnection of the new 400 kV substation ‘D’ with the existing 400 kV substation ‘G’; and other modification works to overhead transmission lines at Jebel Ali.
The new substation will be the first 400 kV substation in DEWA to have a state-of-the-art advanced digital control system. It will be directly linked to the existing 400 kV substation at the ‘G’ station. In addition to exporting power from the new gas turbine units the new substation will also receive power from the future Jebel Ali ‘K’ Station Phase II and export it to the DEWA grid.
On completion of the repowering project, all the steam requirements of the existing steam turbine and desalination plants will be sourced from the new WHRBs which will produce steam from the waste heat of the gas turbine exhaust gas.
Jebel Ali power and desalination station (Station ‘D’) schematic diagram
The station ‘D’ Phase II is located on the Arabian Gulf in the coastal region of Jebel Ali 35 km southwest of Dubai. It consists of a 3 x 75 MW conventional steam power plant integrated with a 3 x 5.72 MIGD desalination plant. The contract for the construction of Station ‘D’ Phase II was awarded in March 1981 and was completed in stages between January and July 1984. The installed capacity of this station is 225 MW of power and 17.16 MIGD of desalinated water from three units.
Gas turbine generators (GT): The new gas turbine power plant is designed to generate approximately 400 MW at 50°C ambient air temperature. The gas turbine generators are capable of sustained operation at their design base rating over an ambient temperature range of 2.8°C to 50°C.
The gas turbines are industrial single-shaft units designed for operation in heat recovery mode. They will normally burn natural gas. Fuel gas will also be provided to the waste heat recovery boilers for firing in duct burners. Diesel oil will also be available although its primary use will be as back-up fuel to the gas turbine. The gas turbine and all associated equipment will be designed to operate continuously and start-up and shut-down on both diesel oil and gas and to run on a mixture of both during fuel changeover.
The turbines are equipped with dual fuel systems and will be capable of burning fuel gas and diesel oil. The gas turbines will be fitted with dry low NOx burners to minimize NOx emissions.
The exhaust from each gas turbine can be vented direct to the atmosphere via a silencer and a bypass stack or to a waste heat recovery boiler (WHRB) by adjustment of a diverter damper. The provision of the bypass stack allows the gas turbine-generator to operate in open cycle mode if the WHRB is out of service.
Waste Heat Recovery Boilers (WHRBs): The WHRBs will be single-pressure units with forced circulation and will provide superheated steam to the existing steam turbines. Each WHRB will have the facility for supplementary firing on natural gas.
With supplementary fuel firing, NOx, SOx, CO and particulate levels in fuel gases leaving the WHRBs will not exceed specified levels. Each boiler installation will have its own boiler stack. The height of the stacks will comply with the environmental standards applicable in Dubai.
Existing steam turbines: The existing Station ‘D’ Phase II comprises three steam turbine units of the extraction condensing type each with a rating of approximately 75 MW. The turbines are equipped with four bleeds for the steam supply to three HP and one LP feedwater heater. The steam to the desalination units is supplied from the controlled turbine extraction steam main from where the deaerator feedwater tank is also supplied with heating steam.
The new WHRBs will be connected to the existing steam and feedwater system to allow individual unit operation of boiler and turbines with associated plants and equipment. In normal operation, the steam turbines will be operated in line with the new GTs and WHRBs, with the three HP feedwater heaters bypassed. Upon a trip of the WHRB, the steam turbine will be shut down and then restarted together with the existing boiler standby unit when the steam inlet conditions to the turbine have been established.
To allow operation of the steam turbine with any pre-selected standby boiler, the existing live steam lines and feed water system will be interconnected.
Control & instrumentation: The entire plant will be controlled from the central control room through an automated sophisticated DCS (Digital Control System).
The repowering project will increase plant efficiency from 30 per cent to 45 per cent. This means the power generated from the steam plant and water production from the desalination plants will be free of fuel cost. The repowering project will be completed in several tranches between 2001 and 2002.
All three desalination units at station ‘D’ phase II are cross-tube double deck multi-stage flash distillers of the brine recycle type with 16 heat recovery stages and three heat rejection stages. Each unit has a distillate output of 26 208 m3 per day at a seawater temperature of 30°C and at top brine temperature (TBT) of 88°C. The plant is capable of producing a distillate output of around 32 370 m3 per day at TBT of 110°C. The desalination plants were designed, supplied, constructed and commissioned by Franco Tosi Industriale S.P.A, Italy, as sub-contractor under the project turnkey contract.
Desalination is achieved by distillation of seawater in a Multi-Stage Flash (MSF) evaporator. This is a widely used method for producing fresh water from seawater on a large scale. The process is based on the principle that the amount of energy which can be stored in water at its boiling point is reduced as the water pressure, which fixes the boiling temperature, goes down. Therefore, when hot seawater (brine) at its boiling point flows into a vessel at a lower pressure, the excess energy from that which can be contained by the water results in ‘flashing’ thereby liberating vapour.
If arrangements are made to condense the liberated vapour using cooling water, pure distillate is obtained and this would constitute a single-stage evaporator. If the brine is then passed into a series of flash chambers, with each chamber maintained at a slightly lower pressure than the preceding chamber, the same phenomena occurs and more vapour is generated. Thus a number of flash chambers are joined together and the technique of vapour production is called ‘multi-stage flashing’.
A basic flash process consists essentially of three sections, the heat input section, which is the brine heater (shell and tube heat exchanger), the heat recovery section and the heat rejection section, both of which are incorporated in the evaporator shell. The steam extracted from the steam turbine or steam from waste heat boiler or auxiliary boiler is led to the brine heater which serves as the heat input section wherein the temperature of the brine is raised to boiling point.
A vacuum is maintained in the evaporator shell by steam operated air ejectors. Seawater discharged from the seawater booster pump is passed through a heat rejection section, where the temperature of the seawater is raised to about 40°C. A part of this seawater is fed as makeup to the heat recovery section through a brine recirculation pump and the remainder is dumped back to the sea.
The heat recovery section consists of cooling tube bundles and a series of flash chambers called stages with each successive stage arranged from higher to lower pressure. Brine from the heat recovery section cooling tube recovering the heat from the condensing distillate flows through the brine heater where the brine is further heated by the low pressure steam coming from steam turbines or waste heat boiler or auxiliary boiler to the boiling point.
The hot brine releases some vapour as it flows through each flashing chamber in the heat recovery and heat rejection sections. The vapour condenses on tubes which carry brine to the heat input section. In the process the incoming brine recovers part of the energy released in the flashing and condensing. Thus it is called the heat recovery section.
The condensed vapour is collected in the trays as distillate. The distillate formed is not fit for human consumption since it lacks minerals and salts required by the human body. The distillate is then passed through a series of limestone filters by the distillate pump, where limestone dissolves in the water giving the alkaline (calcium) hardness to water. To introduce non-alkaline hardness, a small quantity of seawater containing chloride and sulphate is injected into the water at the outlet of the limestone filter. Seawater blended distillate is neutralised by adding caustic soda (sodium hydroxide).
The sodium hypochlorite solution is added before the blended water reaches the water reservoir. The potable water stored in the water reservoir is tested to ensure that it meets the W.H.O. Standards before it is pumped into the Dubai water mains for transmission to Dubai City.
The desalination units have been in almost continuous service since commissioning in 1984, except for relatively short outages for maintenance, overhaul and repair. Recently, the performance of the desalination units has deteriorated leading to a lower distillate production rate.
To improve the performance of all the three desalination units, refurbishment work is being carried out as part of the overall turnkey contract. The desalination refurbishment work is scheduled to be completed along with the repowering ‘D’ phase II project by end of the year 2001.