Two-stroke niche

By the end of August, commissioning is expected to be underway on the new 30 MWe extension to Bahamas Electricity Corporation's diesel-powered Clifton Pier station. This new extension, called DA12, will bring the installed capacity of the station up to some 153 MWe, all in the form of two-stroke diesels.

Systems/plant commissioning on DA12 is scheduled to be completed by the end of September, to be followed by a 30 day hand over period.

The order for the DA12 extension, which uses a MAN B&W 10K80MC-S low-speed two-stroke diesel engine as its prime mover, was awarded in February 2001 to a consortium of Alstom Power SA (Spain) and MAN B&W Diesel's Spanish licensee, Manises Diesel Engine Co, SA.

Alstom Power, consortium leader, is main contractor, responsible for generator, mechanical and electrical auxiliary systems, logistics, site erection and commissioning. The Manises scope includes manufacture and supply of the diesel engines, site erection and commissioning, while MAN B&W is responsible for diesel engine design, foundation design, conceptual plant engineering and commissioning assistance.

The road to Clifton Pier

At the beginning of the seventies, four 10 MW two-stroke units were installed at the Clifton Pier power plant by Bahamas Electricity Corporation.

In 1992 the plant was extended with two 9K80MC-S engines (units DA 9 and DA 10), built by MAN B&W Diesel's Japanese licensee Mitsui Engineering & Shipbuilding Co Ltd and supplied by Burmeister & Wain Scandinavian Contractor A/S, Mitsui's contracting division.

In November 1996 after an extensive international call for tenders, Bahamas Electricity Corporation, decided to extend the plant with a new 30 MW unit.

The prime mover selected was a MAN B&W type 10K80MC-S diesel engine developing 33.5 MW at 102.9 rpm at an ISO efficiency of 49 per cent. The engine was built by Manises Diesel Engine Company and the plant order was awarded to a group of companies led by Alstom Power SA.

The project was financed by the Inter-American Development Bank and the European Investment Bank.

The new plant, called unit DA 11, was successfully commissioned and handed over to the owner, Bahamas Electricity Corporation, in October 1999.

The plant is equipped with a large exhaust gas boiler, utilising the exhaust gas waste heat energy down to some 180 °C. The energy is used for the production of 10 bar steam, partly for heating the fuel oil, but mainly for the production of drinking water, which is being supplied to the local municipality.

During commissioning, extensive measurements were taken of all guaranteed plant values, and fulfilment has been ascertained.

Of course, no plant of such size has been commissioned without teething troubles. In connection with the engine itself, tearing of compensators between turbocharger and air cooler was experienced, as well as repeated accelerated wear on two out of ten cylinders. The difficulties have been investigated in detail, resulting in realignment of the compensators and the introduction of the latest development of ceramic-coated piston rings. Since these modifications were carried out, the engine has been running without any unplanned stoppage.

From its commissioning in October 1999 until 31 December 2001, the engine has accumulated 21 000 running hours and availablility has been over 90 per cent.

The DA12 extension at Clifton Pier will be of identical design to the DA11 plant.

World's largest diesel station

The K80MC-S low-speed two-stroke machine is also the prime mover in the 257 MWe Kilo-X project, the world's largest diesel power plant ever to be built in a single stage, which will be located in Khartoum, Sudan, Africa. Commissioning of the power station will take place in stages in 2004.

The plant will consist of one 7-cylinder engine (7K80MC-S) and six 12 cylinder engines (12K80MC-S).

A consortium headed by the Siemens Industrial Solutions and Services Group (I&S) was awarded a r200 million turnkey contract by DIT Power Kilo-X Ltd (DPKX), of Labuan, Malaysia. The consortium, comprising Siemens and the Polish company, H. Cegielski SA (HCP), Poznan, a licensee of MAN B&W Diesel A/S, will be responsible for planning, engineering and erecting the power station, including all the auxiliary installations, within the framework of an EPC (engineering, procurement and construction) contract.

About half of the contract value is accounted for by Siemens, which will supply and install all of the electrical and automation systems for the power station and the cooling-water supply system as well as for fuel delivery and storage.

A link to Sudan's high-voltage grid will be established by means of a new 110 kV outdoor switching station. In the context of the project, Siemens is also carrying out a study of the power supply network. The results of this study will be incorporated into the design of the plant, which can thus be precisely tailored to the specific requirements of the Sudanese power distribution network.

H. Cegielski is providing the mechanical equipment and supplying the seven engines.

The power from Kilo-X will be purchased by the state National Electricity Corporation, which has its headquarters in Khartoum.

Initially, the power station is to be operated with imported heavy fuel oil but, in the medium term, there are plans to supply fuel from domestic production.

Kilo-X will increase the world population of stationary K80MC-S machines to 25. MAN B&W Diesel's scope at Kilo-X will include diesel engine design, foundation design, conceptual plant engineering and commissioning assistance.

Niche for two-stroke, low speed

As the Clifton Pier and the Kilo-X projects suggest, there is still a niche for large two-stroke low speed diesels in the power generation sector, in particular where reliability and high fuel efficiency are at a premium and where fuel is of poor quality and in short supply. An attraction of two-stroke low speed machines is their tolerance of low grade fuel and ability to run on just about any commercially available liquid or gaseous fuel.

The future development of such engines will be dictated by the market, in particular by future fuel oil prices and quality. The general current trend seems to point in the direction of even more efficient and ever larger units.

While fuel tolerance and fuel efficiency are characteristics of low speed diesels, compliance with environmental requirements is obviously also essential. This has been perceived as something of an Achilles heel for low speed diesels as NOx generation is invariably high from high-efficiency combustion processes.

But technology is available to meet any conceivable regulations, says MAN B&W. Take the case of Guam. Two 40 MW units have been installed on this US Pacific island by Hanjung-KHIC of Korea for the Guam Power Authority. The plant is remarkable for its extensive emission control.

SOx emission is controlled by the use of either high-sulphur or low-sulphur fuel, kept in separate tanks. The high-sulphur oil is burnt when the wind is blowing away from the island, while the low-sulphur fuel is burnt when the wind is blowing towards the island.

NOx emissions are controlled to the level of 950 ppm at 15 per cent O2, as required by the Environmental Protection Agency, by the emulsification of water in the fuel. This technology is well established and has been used, for example, since 1983 in a MAN B&W 20 MW two-stroke engine at Abbot Laboratories on Puerto Rico.
Tables

Power plant performance (Clifton Pier DA11)
Data for 10K80MC-S