Canadians opt for vertical HRSGs at Brighton Beach

6 May 2002

Brighton Beach Power Station (BBPS) is being developed by ATCO Power Canada Ltd and Ontario Power Generation, Inc, as a new natural gas-fired combined cycle generation facility. To be built at the site of the former J C Keith generating station near Windsor, Ontario, Canada, the plant will have vertical gas-flow natural circulation heat recovery steam generators. These are popular in Europe and Asia, but are not commonly used in North America.

The Brighton Beach project is being designed to create new electrical generation capacity to tie into Ontario´s existing power grid.

It is a competitive independent power project intended to compete in the developing open power market in Ontario.

Windsor is strategically located on the Ontario transmission system, and the new facility will strengthen the electricity system in the Windsor area. The facility is situated close to electric power demand, reducing efficiency losses associated with transmission.

The facility will produce electricity for Coral Energy Canada Inc, an unaffiliated independent power marketer, which plans to sell the electricity to interested customers.

The construction of Brighton Beach Power Station was scheduled to begin in 2002 after the receipt of environmental approvals. It will take approximately two years to build, with the commencement of commercial operation in the first quarter of 2004. The operating life of the facility is assumed to be 30 years.

Electricity will be generated by two natural gas fired, 172 MW (ISO) General Electric Frame 7FA combustion turbine generators equipped with low NOx burners. Brighton Beach will have a peak generating capability of around 580 MW and is planned to be capable of baseload as well as frequent cycling operation.

Electricity produced at the facility will tie into Hydro One's existing 230 kV and 115 kV transmission lines in the J Clark Keith Transmission Station adjacent to the new facility.

Vertical HRSGs

Two supplementary fired vertical gas flow, natural circulation (VNC) heat recovery steam generators feeding steam to one 220 MW steam turbine generator will use waste heat from the gas turbines to generate additional electricity.

Vertical heat recovery steam generator technology (forced and natural circulation) is not commonly used on the North American continent but is very popular in Europe and Asia. VNC technology was selected for the Brighton Beach project because of its great suitability for frequent cycling operation.

The VNC HRSGs will be supplied by AE Energietechnik´s HRSG division (being now part of the HRSG business unit within the Babcock Borsig Power group as NEM Energietechnik GmbH) in Austria. The business unit mainly consists of NEM bv of the Netherlands, as leader, Vogt-NEM, Inc, of the USA and NEM Energietechnik of Austria.

The Austrian company developed the VNC HRSG technology in the late 1980s to combine the advantages of vertical forced circulation units with those of horizontal natural circulation systems. In the VNC design, the exhaust gas flow is vertically upwards and the heat exchanger bundles are arranged horizontally. The boiler arrangement is similar to conventional boilers using an external steel structure for top supporting of main components.

Over 50 VNC units, in a variety of sizes up to 270 MW, are in successful commercial operation.

To increase live steam output, the Brighton Beach units have a supplementary firing unit located downstream of the first superheater section, feeding another 45 MW of thermal energy into the flue gas path. The only fuel used will be natural gas from the local network.

To get the most compact layout the gas turbine and the HRSG are connected directly together without employing a flue gas bypass system. This requires a perfectly co-ordinated operation of all major components in line.

To achieve the best performance, the thermal cycle used at Brighton Beach is a triple-pressure reheat system with a cold end preheater cooling down the gas turbine exhaust gas from 599.4°C to 83.2°C at the stack outlet.

Nominal live steam flows and conditions at a supplementary firing duty of 45 MW are as follows: The deaerator is integrated into the LP-system using the drum as a feedwater storage tank.

All of the three pressure levels are equipped with natural circulation evaporators without the use of any start-up devices such as pumps or injectors. The selected design allows the safe operation of the systems. In vertical HRSGs the evaporator consists of the drum, the heat exchanger surface, and the externally routed downcomers and risers. So the circulation is clearly predictable and retraceable.

As far as the operation of the total plant is concerned, full automatic operation is of course applied. Every normal start and stop sequence including trips is managed by the plant control system without requiring a manual input.

Cycling operation

To meet plant requirements, the HRSGs are designed for daily start and stop operation. 50 cold starts and 250 warm and hot starts are expected every year. Consequently, it is essential that start-up times are as short as possible and quick restart capability is ensured.

In order to operate the plant successfully, all important plant parameters such as steam temperatures and pressure levels, pinch and approach points, and flue gas pressure losses are optimised technically and economically. For example, the HP pinchpoint is 7°K and the total static flue gas pressure loss is at 26.7 mbar in the design case.

One of the major demands that the plant must contend with is daily cycling capability. As the heat recovery steam generator contains certain key components requiring very high quality materials with large wall thicknesses, considerable effort has gone into making sure that the boiler design concept is fit for such a cycling regime over the whole lifetime of the HRSG.

Generally the vertical natural circulation boiler design (VNC) with its external steel structure and top supporting system is very flexible and the free hanging heat transfer bundles compensate for the thermal expansion during transient load conditions. Drums are located above the top grid and they are connected to the bundle headers via the externally routed pipework. The dimensioning of the parts is generally standardised but where more stringent conditions require a lower stress level, adaptations can be made.

The heat transfer bundles are designed to have a U-shape with headers on the inlet and outlet as well as return bends on the opposite side.

The finned tubes, mostly serrated, are allowed to slide in their supports and expand freely in the axial direction. The thermal flexibility achieved through this design approach is supported by other measures such as minimising the wall thicknesses of heavily loaded pressure parts by using high quality materials, such as T91 and P91, and minimising diameters while strictly following the ASME code.

As far as welding is concerned, further quality related decisions have been made.

In such a heat recovery steam generator, all connecting welds between tubes, header stubs and pipes are fully penetrating butt welds and all the header stubs are aligned radially in order to get the best possible welding conditions. The welds between the header tube and the stubs are full penetration fillet welds. Appropriate welding preparation is performed on both header and stub side.

The number of welds on the headers is reduced to a minimum because X-ray testing of such welds is not possible.

Measures such as omission of economiser partition blades as well as the generous sizing of the superheater and reheater drains to avoiding inhomogeneous cooling of the headers during trip and restarting phases are also included in the design.

As well as the pressure parts, the need for daily cycling also had to be taken into account in the design of the boiler casing and stack. The casing is internally insulated with an inner lining, so thermal expansion and stresses are minimised. The boiler hood, carrying a "cold" silencer is insulated on the outer side and right at the stack inlet a flue gas damper is located in order to keep the HRSG hot during shut down phases.

The stack is made of CORTEN steel because the appearance of condensate is expected to be frequent due to the crossing of the water dew point during start-up and shutdown phases in the cold seasons of the year.

Summarising these design measures, it can be seen that the need for daily cycling duty has been a major influence on the boiler design. The end result is confidence that pressure parts which are sensitive to creep fatigue will continue to operate trouble-free during their lifetimes.

Nominal live steam flows and conditions at a supplementary firing duty of 45 MW readings

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