MSF innovation reduces energy loss and emissions

SCIPCO, a consortium of CMS Energy and International Power, is developing the 1500-MW and 120-mgd combined cycle power and desalination plant at Shuweihat, 250 kilometres west of Abu Dhabi. SCIPCO will operate the plant under a power and water purchase agreement with the Abu Dhabi Water and Electricity Authority (ADWEA). SCIPCO will also own 40% of the plant on completion, with ADWEA holding the balance. The power and desalination facility is scheduled to enter full commercial service in 2004.

Two engineers with PB Power (a division of Parsons Brinckerhoff), which acted as developer's engineer and subsequently as owner's engineer for SCIPCO on the Shuweihat project, invented a modification to the standard multi-stage flash (MSF) distillation process that will be used in the plant. The patented process enhancement, developed by Paul Willson and George Atkinson, is believed to be the first fundamental change to the MSF process since its invention in 1957 by R. S. Silver.

Desalination of seawater to provide potable water is widely used in many Middle Eastern countries where it is the major source of freshwater. Thermal processes produce most desalinated water in the region. These boil seawater to drive off water vapour that is then condensed as freshwater. Although modern processes, such as MSF, recycle the heat involved in this boiling process, they remain energy-intensive and normally use low value heat from a power plant that has limited potential for further power production.

The PB Power innovation modifies this conventional combination to minimise the losses associated with interfacing an MSF unit to a power plant. The key interface is the return of steam condensate to the power cycle.

High performance MSF units heat seawater to around 110°C, which, allowing for temperature losses in heat exchangers, means that condensate from the steam supplied from the power cycle is returned at about 115°C. This prevents the power plant heat recovery steam generator (HRSG) from recovering energy from the flue gases below a temperature of about 140°C. This represents a significant loss compared with a combined cycle gas turbine power plant, where flue gas temperatures are typically 95°C.

The new process modification cools the steam condensate from the MSF unit, allowing "free" heat in the flue gas to be captured, while recycling the heat from the condensate into the distiller. This reduces the steam demand of the distiller by up to 6% and replaces it with energy that would otherwise be wasted. The result is an overall fuel saving of up to 2% with corresponding reductions of power plant emissions.

The heat removed from the condensate, which is ultimately recovered from HRSG flue gases, is returned to the MSF distiller by recirculating a small proportion of the freshwater produced through a condensate heat exchanger. The heated freshwater recirculation is added to the flow of water produced at hot end of the distiller. This additional heat enhances the warming of the seawater by the condensation of vapour in the distiller, reducing the demand for steam to heat it to the top temperature.

On a large plant such as Shuweihat, fuel savings worth in excess of US$ 1 million per year are possible, according to Willson. "This enhancement significantly improves the performance of new and existing plants for minimal additional capital outlay. It also uses conventional technology and operating conditions, making it extremely attractive to owners," Willson said.

"The costs of enhancing the cycle are low, with the additional equipment on the distiller representing a cost increase of much less than 1%," Willson said. "The adjustments to the power plant design are close to cost-neutral as steam generator evaporator, drum and superheater capacities are reduced while lower-cost economiser capacity is increased," he added.

Willson and Atkinson believe that the improvement in fuel consumption of up to 2% at low initial cost, with minimal impact on the MSF distiller will make the new process attractive to owners and developers of power plants, particularly in the Middle East, where the MSF process is widely used.

Notes:
Willson and Atkinson won the British Energy Award for Sustainable Engineering in 2001 for their invention. Paul Willson is the deputy director of engineering at PB Power, located in Manchester, England. George Atkinson is the engineering manager, also at PB Power. For more information, contact [email protected] or [email protected]