G A Atkinson and P M Wilson, PB Power, UK

Modern desalination processes are well-proven but are highly energy intensive. An innovative desalination technology development is now promising to improve the economics of major dual-purpose power and desalination plants.

The combination of CCGT and multistage flash desalination processes has become a common and successful arrangement in dual-purpose power and water plants in the Middle East. Plants of this arrangement have spearheaded the introduction of independent water and power projects (IWPP), resulting in much reduced water and power production costs in a region where demand for electricity and water is growing rapidly.

In this competitive environment, the UK’s PB Power, acting as developers’ engineer to project developers CMS Energy and International Power, introduced an innovation to the 1500 MW and 100 million Imperial gallons per day (MIGD) Shuweihat IWPP in Abu Dhabi, UAE, contributing to the plant’s exceptional levels of efficiency (see pages 47-49 for more information on the Shuweihat project). The innovation has now been granted patents internationally.

The innovation is a variation on the standard multi-stage flash (MSF) desalination process, and it is believed to be the first fundamental change to the basic process since its invention almost 50 years ago.

Conventional cycles

The desalination of seawater to provide potable water is a widely used process, and for many Middle Eastern countries is the only source of fresh water. Although modern desalination processes such as MSF are highly regenerative, they remain energy intensive and make use of low-grade heat from an associated power plant.

The design of dual-purpose power and water plants is similar in many respects to combined cycle gas turbine (CCGT) power plants in which gas turbines exhaust to heat recovery steam generators (HRSGs) with the steam raised being expanded in a steam turbine to produce additional power. However, the power cycle of a dual-purpose plant differs in that most, if not all, of the steam raised in the HRSGs is expanded in the steam turbine to about 3 bars before it is used in desalination, which uses a thermal distillation process to produce potable water from seawater.

In a MSF desalination plant, the steam from the steam turbine is supplied to the brine heater of the MSF plant, typically heating brine to around 110°C.


Figure 1. The basic MSF desalination process is commonly used in desalination plants all over the world
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The process includes three main sections (see Figure 1):

  • The brine heater (left)
  • The heat recovery stages
  • The heat rejection stages (right).

Hot pressurised brine (at 110°C in this example) leaves the brine heater and enters a series of chambers (or stages) at successively lower pressures. The pressure reduction causes the brine to flash, releasing water vapour, which is condensed on cooled tubes at the top of each stage. The condensed, distilled water is collected in a product trough, which cascades through the plant from the higher to lower pressure stages leading to the product pumps.

The flashing brine flow on the floor of the distiller (flowing from left to right in Figure 1) is progressively cooled as vapour is flashed off at each stage. Some of this brine is discharged to waste to control the cycle concentration levels.

The condenser tubes in each of the heat recovery stages are cooled by recycled brine taken from the lowest pressure stage, supplemented by make-up seawater. This flow is progressively heated as it passes up the distiller (from right to left in Figure 1) by the condensing product vapour at each stage. By the time the recycled brine reaches the inlet to the brine heater it is typically only a few degrees below the Top Brine Temperature (TBT). In the example shown, a temperature rise of only 7.1°C is needed in the brine heater to attain the TBT of 110°C.

Heat is removed from the distiller primarily in the lowest temperature heat rejection stages, where the cooling medium is an open cycle flow of seawater.

A limitation of the conventional MSF cycle is the high temperature of the brine heater condensate, typically 113-115°C. This is much hotter than condensate returned to the HRSG in a CCGT cycle, increasing stack losses significantly for such a dual-purpose plant.


Figure 2. The improved MSF desalination process results in direct fuel savings for desalination and dual-purpose plants
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In response to this and to the increasing demand for dual purpose power and desalination plants, PB power has developed a modified MSF cycle designed to reduce stack losses. The new technology can be applied in new plants such as Shuweihat, and can also be retrofitted to existing distillers to provide benefits in steam consumption levels.

An improved process

The aim of the improved CCGT and MSF process is to return condensate to the power cycle at a substantially lower temperature while using the heat recovered beneficially within the distillation process.

The revised process (Figure 2) retains all of the features of the conventional process but incorporates a small heat exchanger in the brine heater condensate return.

The cooling medium for the new heat exchanger is a small part of the product water flow from the product trough in the last heat recovery stage. The product recycle flow heated by the hot condensate is returned to the product trough of the top stage of the heat recovery section. The additional product flow cascades through the heat recovery stages, flashing and condensing in the same way as the rest of the product flow. The additional heat is therefore recovered to the recycle brine flow, increasing its temperature rise through the heat recovery stages.

In the example shown in Figure 2, the brine recycle temperature leaving the heat recovery stages and entering the brine heater is increased by 0.4°C compared with the original value. This reduces the required temperature rise in the brine heater to 6.7°C compared to the previous 7.1°C, cutting the heat demand of the brine heater by about six per cent.

In other respects the small changes of temperature and heat flow introduced by the condensate cooler mean that it has minimal impact on the distiller operating conditions.

Optimised cycles

The process modification has two principal effects on the power production cycle.

Firstly, the reduction in steam required by the brine heater allows the level of supplementary firing in the HRSG to be reduced. This modification therefore generates direct fuel savings and reduces emissions.

Secondly, the energy required to heat the cold return condensate is obtained from the boiler stack gases, reducing the stack loss. This more effective use of the energy in the HRSG improves cycle efficiency.

For a combined cycle plant, a direct fuel saving of between 1.5 and 2 per cent is generally possible with the improved MSF technology. This represents a significant financial advantage with a secondary benefit of a corresponding reduction of atmospheric emissions.

For a plant of the scale of Shuweihat the value of fuel consumed annually, even at the low gas prices prevailing in the Gulf, is close to $100m. Hence a reduction of 1.5 per cent in fuel consumption represents a valuable saving. Discounting the value of this saving using assumptions typical of IWPP evaluation gives a present value of $12.8 million and a net present value of around $10 million, allowing for the small additional capital cost. The corresponding reduction in carbon dioxide emissions is 80 000 t/y.


Figure 3. The Shuweihat S1 power and desalination plant in Abu Dhabi will be the first plant to use the improved MSF desalination process. The plant is scheduled to start operation in 2004
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Similar applications to existing plant offer valuable fuel savings but may involve additional costs for modification to the HRSG to handle the reduced temperature of condensate return. Nevertheless such applications can be both feasible and economically attractive.

Implementation

The condensate cooler is a simple addition to the basic MSF distiller. Apart from the small external heat exchanger, low capacity recirculation pump and some piping, there is no significant change to the distiller design.

The condensate cooler has the advantage that it can be switched on or off with little impact on the distiller operation, apart from its steam consumption. For this reason the additional equipment need not be duplicated to maintain reliable production.

The cost of the heat exchanger and associated equipment is low, typically adding much less than one per cent to the cost of the distiller. The HRSG requires a larger economiser, but this is substantially offset by the smaller size and cost of the high pressure evaporator and superheater. The overall project cost increase is very small.

The condensate cooler concept patented by PB Power represents a significant development in the design of dual-purpose power and water plants, contributing to reductions both in tariff costs and in environmental discharges.

The technology won the British Energy Award for Sustainable Engineering in the UK Engineering Council Environment Awards in 2001. PB Power is in discussion with plant owners and developers in the Middle East interested in the benefits of this technology.