Merchant of Saltend

Junior Isles

Every so often a project comes along which captures the imagination. With its size, flexibility of operation and innovative financing; Saltend is one such project.

The Saltend cogeneration project is a 1200 MWe nominal combined cycle cogeneration power plant being constructed at Hull, in the UK. The cogeneration plant will provide process steam to the British Petroleum Chemicals manufacturing Saltend complex located adjacent to the facility.

Equally importantly, electricity from the plant will be sold into the national grid on a merchant basis.

Project origin

The origin of Saltend dates back to early/mid-1997 at which time BP Chemicals Saltend was looking at its electricity and steam needs for the site. At the same time, US IPP, Entergy, was looking at opportunities in the UK power market with the onset of competition and deregulation.

BP Chemicals was looking for a way to lower its electrical power cost. It looked at its existing site since it would allow the better control of costs. There was a bidding process for the plant soon after which Entergy entered into a contract with BP to provide a specific amount of power and process steam. Entergy entered into a separate 15-year contract with BP Gas Marketing for the supply of natural gas.

The plant is being jointly developed by BP Chemicals and Entergy. The US company holds 100 per cent equity in the project.

One of the most notable aspects of the project is its financing. The project represents the world`s largest non-recourse financing for a merchant plant – representing a $1.2 billion non-recourse loan package.

United Bank of Switzerland was the financial advisor and lead arranger for the financing. The company underwrote 100 per cent of the non-recourse loan package. Other investors were then sought after this.

The project is not significantly different to a conventional IPP arrangement in terms of the agreements with BP. However, the agreement does allow Entergy to sell the majority of the plant`s output into the grid through Entergy`s trading and marketing experts in London.

Saltend is largely a merchant plant but has a 15-year power purchase agreement to sell about eight per cent of its maximum electrical output to BP Chemicals. The rest will be sold to the grid where there are price and offtake risks. There is also a 15-year steam purchase agreement to supply BP Chemicals.

Plant design

The plant is being built by Raytheon Engineers and Constructors UK Ltd. (Raytheon) for Entergy Power Group (Saltend Cogeneration Company Limited). Raytheon, as leader of the consortium with Mitsubishi Corporation, will engineer, design and supply the Balance of Plant (BOP) equipment, as well as perform all construction, start-up and commissioning activities. Entergy has a maintenance contract with the Mitsubishi team.

When selecting the technology for the plant, Entergy took into account a number of different criteria: the use of fuel which was available at a reasonable price; environmental performance; and plant efficiency were all key issues. This led to the selection of a combined cycle arrangement using proven technology. The plant will in fact use the same technology as Entergy`s Damhead Creek project.

At the design ambient pressure of 1015 mbar, ambient dry bulb temperature of 10.3 degrees C and relative humidity of 80 percent, with one per cent boiler blowdown; the facility should generate 1175.3 MW net output with a net rate (LHV) of 6486.7 kJ/kWh. The exhaust gas temperature of the gas turbine is 586 degrees C.

The facility will use 3 x 701F2 Mitsubishi Heavy Industries (MHI) gas turbines in a single shaft combined cycle configuration, each with a three pressure level vertical HRSG and a reheat steam turbine. The facility will consist of three combined cycle gas turbine power train modules, each module containing one gas turbine, one steam turbine and one generator, assembled on a single shaft with one heat recovery boiler and associated ancillary equipment.

The design and construction of the facility shall be suitable both for continuous base load operation, and for intermittent or part load operation, at a constant or fluctuating level.

The gas turbines are dry low NOx units which operate with controlled combustion to keep NOx to a minimum. Guaranteed NOx limits from the plant are less than 60 mg/m3; and CO limits are less than 100 mg/m3.

Each turbine is directly coupled to a three-pressure vertical natural circulation HRSG. There is no bypass stack. Because it is a single shaft configuration, the unit uses steam from a startup boiler to spin the steam turbine which is used as a starting device to spin up the gas turbine to about 20 per cent of speed. At this point ignition is initiated and the gas turbine continues to accelerate to a sustaining speed.

Heat from the gas turbine is input to the HRSG. The HRSG is a natural circulation unit with no forced circulation pumps. This is fairly unique for a vertically designed boiler. This is useful during startup since the unit does not have to have circulation pumps running. It has an integral deaerator which initially takes steam from the startup boiler but then starts getting steam from the LP steam section so it is self sustaining.

From a cold start, the steam initially developed in the HRSG is too cold and too wet for input to the steam turbine. The steam turbine therefore continues to operate on steam from the startup boiler while the wet steam from the HRSG is fed to the condenser. The condenser is a titanium tubed condenser which is not only designed for condensing the steam from the steam turbine but also to take dump steam during startup and load rejection cases.

When the HRSG has warmed up sufficiently so that the steam is of good quality, the digital control system automatically switches operation from the startup boiler to the HRSG to produce additional power in the steam turbine. Once the steam turbine is on line, taking steam from the HP, IP and LP sections of the HRSG, the turbine controls release the gas turbine to ramp up at adjustable rates to the specified load.

In the HRSG at full load, HP admission steam is 105 bar/538 degrees C, cold reheat steam is 35 bar at 384 degrees C; hot reheat steam is 31 bar at 538 degrees C; LP steam is 5 bar at 250 degrees C.

The single shaft configuration offered the advantages of a rapid startup and there is also a cost saving since there is one less generator and one less step up transformer per module.

Each module has an output of about 400 MW at the design conditions. Steam output is up to 150 t/h and this can come equally from the three modules or supplied from a single module. It is envisaged that during most operation, the steam will be supplied equally by the three blocks for best efficiency.

The modules are controlled by a digital control system which interfaces with the gas and steam turbine control systems. These systems give the operator full control of how steam is exported to the plant and how he wishes to operate each of the gas turbines.

In terms of electrical output, the plant will bid into the grid in half hour increments. If prices are favourable the plant could provide nearly 1200 MWe. If prices are not favourable, the plant could run to a much lower level.

There is also a secondary boiler which can make steam for the host even if electricity is not being generated. This is a conventional single pressure gas fired package boiler.

Because of the required flexibility, each module is essentially a standalone unit which can run independently of the other two units. Once the first unit is started, the second unit can be started with bleed steam from the running unit.

A unique feature of the plant is that it only has two sets of cooling towers for three units. This is due to space constraints at the site. There are 30 cells in the cooling tower organised in two basins of 15. In one basin, the front ten cells are for the first module; in the second basin the front ten cells are for the second module; while the last five cells of each basin are for the third module.

This called for careful layout of the circulated water piping and equalization channel to allow water to transfer from one basin to the other. This means there are three sets of circulating water pumps: one pair for the respective set of cooling cells for each module.

The primary source of cooling water make-up to the facility will be saline water from the nearby King George dock. The source of water for all other consumption on site, i.e., domestic, potable, demineralized feed and fire water shall be from Towns water supplied by Yorkshire Water. Waste water from the facility will be treated and re-used to the maximum extent.

Environmental concerns such as these played a big part in the project development and Entergy and BP worked with environmental groups closely throughout. This was especially important since the Humber Estuary is one of the primary bird nesting transitory areas and the project partners wanted to ensure that migratory patterns were not upset during construction.

Operation

Optimization of the plant is an important point. As with most cogeneration applications, there is a dedicated steam host which is always given first priority, especially when there is a contractual arrangement. At Saltend, the steam load is designed to produce up to 150 t/h, although the steam load will remain relatively fixed. The electrical output from Saltend can be anywhere between 240MW and 1200 MW. In terms of the optimization point, at 100 per cent load each unit will produce about 40 t/h of steam and 400 MW of electricity.

BP can also nominate steam on a half hour basis with roughly 24 hours advanced notice. As grid conditions change the plant will automatically reconfigure through the digital control system to keep delivering the steam while varying the electrical output. The plant is optimized over a wide range and can be run so that almost no electricity is generated from the steam turbine.

The gas turbines step up to 275 kV for connection to the National Grid Company switchyard. There are a pair of step down transformers to BP Chemicals to step the voltage down to 33 kV for use in the chemical plant. There are provisions which allow a selected unit to operate in `island mode` so that BP can be supplied at all times.

Project schedule

The engineering consortium received full notice to proceed on December 14, 1997, thereby initiating this fast track 25-month schedule. Ground breaking for the plant took place soon after and the plant is said to be on schedule and within budget. The project is scheduled for handing over on January 24, 2000.

During the construction process, some 600 people are employed at the site. Once complete the plant will employ about 40 people.

The plant is now nearly 40 per cent complete (as of January 1999). Major civil construction activities are currently about 80 per cent complete and mechanical and electrical construction activities well underway.

All of the major equipment has been ordered with significant deliveries to site which include gas turbines No. 1 and 2, main and auxiliary transformers, and startup and secondary boilers. Engineering was over 90 per cent complete as of January 29, 1999.

Click here to enlarge image

Figure 1. Aerial view of the Saltend site. Photo courtesy of S.G.A.P

Click here to enlarge image

Figure 2. Erection of the inlet air duct support frame

Click here to enlarge image

Figure 3. Unit 3 main transformers

Click here to enlarge image

Click here to enlarge image

Click here to enlarge image

Figure 4. Construction activities at the startup boiler

Click here to enlarge image

Figure 5. Unit 2 boiler structural steel work