Five fully modularised vertical once-through-boilers (OTBs) – designed, engineered, manufactured and installed by John Cockerill Energy – are being employed at the Bridge Power combined cycle plant in Ghana, which is currently being commissioned, with commercial operation expected by August 2021.

The facility, which runs on LPG, with distillate oil as back-up, is being constructed in three stages. The first stage (Stage 1a) was installation of five fast-deployment GE TM2500+ (truck-mounted) aeroderivative gas turbines, each rated at 33.5 MW ISO, running in simple cycle mode. The second stage (Stage 1b) consisted of conversion to combined cycle, with the addition of the five OTBs downstream of the gas turbines plus a 52 MW steam turbine.

Steam conditions are 512°C/64.8 barA for the high-pressure and 232°C/4.6 barA for the low-pressure loop.

A Stage 2 is planned, which would see total installed capacity increased to around 425 MW, for completion in 2023.

This new combined cycle generating capacity is long awaited in Ghana to reduce power shortages and frequent power cuts, including a recent countrywide blackout.

The Bridge Power facility, which is expected to operate in baseload mode, with periodic cycling, required maximum operational flexibility and an extremely compressed construction schedule. The small modular once-through HRSGs proved to be an ideal choice for Stage 1b, with their capability (with deployment of appropriate materials) to “run dry” – ie, to keep the gas turbine running when steam generation is not required – which decouples the gas turbines from the steam cycle.

The prime contractor for Bridge Power is GE, with boiler erection carried out by Metka on behalf of GE.

The HRSG pressure parts were manufactured in modular box style in Korea. The modular pressure part box OTB design maximises shop fabrication by enabling 100% of the pressure parts to be pre-assembled in a single module.

Benefits of OTB

John Cockerill Energy offers OTB for all HRSGs behind gas turbines of less than 100 MW. OTB is seen to offer key advantages over drum technology (see Pascal Fontaine, “Modular once-through boiler for flexible small-scale combined cycle”, Modern Power Systems, June 2020, pp 14-16).

Rather than having separate tube banks for economiser, evaporator and superheater, the once-through boiler performs all these functions in a single serpentine circuit from economiser inlet to superheater outlet. Water is heated, vapourised and superheated within this single tube bank and the sections grow or contract depending on heat load.

Key benefits of the OTB include: improved thermal cycling capabilities thanks to the “drumless” design; superior tube metallurgy; “dry running” capability, as already noted; simplified controls; superior operational flexibility; and enhanced modularity and constructability.

Among the potential market opportunities envisaged by John Cockerill are the following: new combined cycle power plants; conversion from SC to CC (compact footprint, vertical arrangement, no bypass stack); flexible power projects (daily cycling); decentralised power (smaller power plants); cogeneration / CHP (industrial & district heating); integration of CC and renewable generation; and offshore applications: FLNG, power barges, offshore platforms.

In short, the vertical OTB promises lowest lifecycle cost, deriving from flexibility, reliability, and simplicity.

However, the OTB requires demineralised and polished feedwater, with a cation conductivity limit of 0.25 μS/cm for a small HRSG to be maintained continuously. This incurs additional costs relative to a drum boiler, although it should be noted that the presence of a condensate polisher and cleaner water has benefits throughout the plant, eg ensuring a supply of high quality steam to the turbine, reduced boiler maintenance and extended boiler life, elimination of need for blowdown and reduced need for make-up water, less maintenance and downtime, and elimination of tube scaling, deposits and carry over.

The OTB experiences no efficiency loss due to bypass stack damper leakage, and no maintenance is required on a diverter damper, with less piping and E&I scope, while simplified operation minimises operating costs.

Some other OTB salient features can be summarised as follows:

  • Unrestricted GT start up without holding time from steam cycle.
  • Fast steam cycle start up and shut down capability.
  • A good fit with intermittent renewable energy.
  • Dry running OTB with properly equipped SCR system provides good control of GT emissions in open cycle.
  • Steam temperature controlled by feedwater flow.
  • Ability to operate down to 30% of GT load while maintaining steam temperature.
  • Low thermal inertia, with thin wall pressure parts throughout and elimination of the thick wall drum.
  • Thin-wall tubes and no limitation on pressure gradient compare favourably with limitations created by of thick-walled drum.
  • Efficient off-design performance: steam temperature can be controlled by the feedwater flow with or without the need for desuperheating.
  • Improved capability for controlling emissions during start up: the SCR is quickly heated up to operating conditions.

In summary, the following advantages can be claimed for the OTB: lowest boiler capital cost on a supply and install basis; lowest lifecycle cost due to the simplicity of the product; operational flexibility for today’s demanding operating conditions; smallest footprint and least weight, making the technology suitable for floating power and conversions to combined cycle; and ability to dovetail with intermittent renewables and peaking plants.

Floating LNG

While John Cockerill Energy favours the OTB for small HRSG applications, other configurations are also available, for example completely modularised vertical drum type HRSGs, as supplied by John Cockerill to Golar LNG’s floating LNG facility (Golar Hilli GoFLNG), currently in operation off Cameroon, West Africa – the world’s first floating LNG plant.

It employs four HRSGs in conjunction with four GE LM2500 G4 aeroderivative gas turbines (34 MW each) plus two steam turbines in combined cycle mode to power the gas liquefaction process, with a total installed capacity of some 196 MW.

The vertical HRSGs employ a cold casing design with assisted circulation.

On-board HRSGs can be either of the drum type or employ once through boiler technology. In the case of a drum boiler, assisted circulation is preferred to render evaporator circulation immune to ship-board motion, as well as enabling a more compact arrangement and reducing weight.

OTB is certainly the best fit for on-board applications. Indeed, the origins of the once through boiler concept can be traced to the 1980s US DOD/US DOE/Solar Turbines Racer Program, which required an HRSG for offshore combined cycle applications (to provide additional ship propulsion power) that was lighter, smaller, simpler and unaffected by the pitch and roll of the seas.

Nevertheless, in the case of the Golar Hilli project, the EPC contractor, Black & Veatch, opted for a vertical drum type HRSG.

The Golar Hilli project was developed under an agreement between Golar LNG Ltd, owner and operator of LNG carriers, Société Nationale des Hydrocarbures (SNH)  and Perenco Cameroon for a floating LNG production facility to develop gas fields located off the Cameroon coast for export to Asia. The region is conducive to FLNG as it produces comparatively clean, dry gas under moderate ocean conditions.

Golar’s floating LNG facility is enabling the monetisation of what would otherwise be considered a marginal field, allowing a once “stranded” gas field to enter the global LNG marketplace. When this field is depleted the FLNG can quickly reposition itself within another field and continue production.

The project was also the world’s first conversion of an existing LNG carrier (Golar Hilli) into an FLNG, which offers a potential solution to the problem of what to do with relatively small and less economic 1970s-vintage LNG carriers.

Singapore’s Keppel Shipyard and Black & Veatch partnered with Golar on the conversion of Golar Hilli.

A big advantage FLNG has over conventional field development is the conversion platform can be delivered at least a year earlier than a new land-based infrastructure build, which gets LNG to market sooner. Also, first cost of the liquefaction plant is reduced because the entire LNG facility is built and tested in the shipyard before moving to its operating location.

Black & Veatch, as EPC contractor, was responsible for the design and engineering of the topside LNG system, which uses Black & Veatch’s PRICO (poly refrigerant integrated cycle operation) liquefaction technology. The process provides several key advantages, including a simplified refrigeration system design that minimises equipment and reduces footprint, making it ideal for offshore liquefaction conversions, employing one kind of mixed coolant (a mixture of methane, ethane, propane, pentane, and nitrogen) and one large heat exchanger.

The liquefaction process requires massive amounts of power to run the gas compressors and prodigious amounts of process steam at very high levels of availability and reliability.

The four gas turbines are located under the main deck and each exhausts vertically into the John Cockerill single-pressure HRSGs located on the upper deck. The configuration of the inlet duct thus represents a major departure from typical onshore HRSG design practice.

The HRSG footprint is minimised by using assisted circulation, as already mentioned, rather than natural circulation. This means the water is pumped through the evaporator tubes rather than being driven by density differences as in a natural circulation HRSG.

The Golar Hilli HRSGs each produce 39.1 t/h of steam at 45.3 bar and 399°C. The total output is sufficient to produce 30 MW in each of the combined cycle system’s two steam turbines as well as the steam required by the liquefaction process.

Modular design of the entire liquefaction and power generation systems was a necessary requirement for meeting the tight renovation schedule. Each HRSG was manufactured in two large assemblies, with the larger one weighing 240 t.

John Cockerill carried out studies on the marinisation of the HRSGs, taking into consideration the extreme movements to be encountered at sea, eg, roll, pitch and acceleration. The studies resulted in some design changes, including compartmentalisation of the steam drums and deaereators to prevent water “sloshing” and to maintain stable boiler operation.

For further information, contact: John Cockerill Energy (; Pascal Fontaine,; Caleb Lawrence,