Designer HRSG fits the bill when it comes to repowering

In Russia, there is a large fleet of old coal fired power plants ripe for repowering into modern combined-cycle power plants. This article reviews the specific HRSG design features for such projects, many of which will have site constraints, using Singapore’s Senoko repowering project as an example.

Pascal Fontaine, CMI Energy, Belgium

On the 1 July the United Energy System of Russia (RAO UES), the state-owned utility, ceased to exist as a legal entity, and marked the end of a five-year reform plan, which has seen Russia’s electricity sector transformed from a Soviet-era monopoly to one comprising six wholesale generation companies and 14 territorial generation companies; many of which now partly owned by foreign utilities.

Based on an annual demand growth forecast of five per cent, RAO UES announced earlier this year a new five-year programme involving the construction of an additional 40.9 GW of generating capacity. The goal to add up to 160 new power units, with an aggregate capacity of nearly 30 GW by 2010.

The Senoko plant was successfully repowered into a efficienct combined-cycle plant, with an output capacity of 1080 MW
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Russia, however, has a large fleet of old coal fired power stations, which have the potential to be repowered into modern combined-cycle power plants, and therefore make a valuable contribution to the country’s demand for greater capacity.

A fact that is not so widely known is, that in many old conventional plants although the coal fired boilers have exhausted their useful life, in many cases, the steam turbines have not. Thus, repowering these old steam sets into efficient combined-cycle with new gas turbines and heat recovery steam generators (HRSGs) can be a more cost-effective solution thatn building a new plant.

However, a repowering project is rarely as straightforward as a greenfield project, where ‘reference plant’ concepts can be applied. Repowering projects in most circumstances need to fit within the existing plant layout and meet the steam turbine operating condition.

One such example is the Senoko repowering project in Singapore, where the vertical HRSG had to be adapted and modeled to match an extremely limited footprint.

Repowering gives new lease of life

Senoko Power, Singapore’s largest electricity generator, completed a large-scale engineering project for the combined-cycle conversion of an old steam plant that was powered by three 120 MW oil fired boilers that were approaching the end of their operating life.

To give the plant a new lease on life, Senko decided to convert the plant to combined-cycle by removing the old boilers, replacing them with gas turbines and heat recovery steam generators, and modifying the existing steam turbines to operate at higher steam conditions and flow.

Each of the 120 MW steam sets was converted into a 360 MW combined-cycle unit. All three blocks incorporated Alstom GT26B gas turbines, which exhausted into a CMI vertical-type HRSGs. The latter had to be custom designed and installed to meet site space restrictions and project time constraints.

Two-phase project worked well

To ensure that plant outage time and operational disruption were kept to a minimum, the units were repowered in two phases. The first phase allowed for a single 360 MW combined-cycle conversion, which was followed by the development of the other two blocks together.

During the first phase, the gas turbine and HRSG were installed alongside an operating boiler to keep the old steam turbine plant in service as long as possible. Once that part of the conversion was completed, the unit was shutdown for about 18 months to allow for the refurbishment of the existing steam turbine and connecting up the balance-of-plant systems. Throughout this period the other two units remained in operation to generate power for the grid.

Upon completion of the first phase, it was possible to remove the remaining two units from service because the combined-cycle generating capacity of the refurbished unit was now more than double that of the other two smaller steam sets combined. Their combined-cycle conversion was subsequently undertaken within a narrow time window of 18 months, with the complete plant being returned to full commercial service on schedule.

Alstom was the project leader and awarded the HRSG contract to CMI. The HRSG design was a vertical natural circulation with three pressure levels plus reheat: HP, 322 t/h at 128.9 barA, 568 à‚°C; IP, 27.9 t/h at 41.5 barA, 320 à‚°C; LP 17.8 t/h saturated at 5.4 barA, 237 à‚°C ;and RH, 332 t/h at 39.4 bar 568 à‚°C.

Challenge of space constraints

At Senoko, the contractors were faced with severe space limitations because the new building which would house the gas turbine, HRSG and all steam piping and connections to the steam turbine could not exceed the footprint of the original boiler. The building also had to include the auxiliary equipment such as feedwater tank, pumps, sampling and dosing skid, plus a cargo lift, creating significant system-interface design risks.

The hydraulic trailers had an essential role in the successful erection of the modular-designed HRSG
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The steam generator presented the biggest challenge. To accommodate the space limitations, the gas turbine exhaust had to be extended into the boiler enclosure by about a metre, which reduced the available area for each HRSG to 30.6 m in length by 28.1 m in width. Thus, at an early stage in its design, CMI developed three-dimensional modeling of the HRSG (including all auxiliaries) and worked with Alstom to identify and resolve any potential interface problems.

Design flexibility of HRSG

A standard vertical-type HRSG for a class-F gas turbine combined-cycle plant typically has an overall length of around 35 m. This length is based on a tube length of 20.4 m, which is the longest length available for a CMI vertical design, and the standard for new installations with no space limitations. However, that size exceeded the on-site limits imposed at Senoko.

Unlike horizontal HRSGs, the boiler tube length impacts directly on the boiler length of vertical HRSGs, so CMI engineers took advantage of this flexible lateral and vertical arrangement and made a design trade-off by shortening the tube length and enlarging the boiler casing width. In addition to meeting requirements for physical dimensions, it was also essential to ensure that the gas pressure drop did not change, i.e. the gas velocity and the gas path cross-sectional area remained constant. It is important to keep in mind that gas pressure drop is proportional to the square of gas velocity in an HRSG. It is a critical parameter that should be guaranteed by the HRSG supplier, and highly sensitive to available gas path cross-section. As a consequence of increasing the casing width the gas path was divided into three sections instead of two, as in the standard HRSG design.

A total of 12 heat exchanger modules were shipped, offloaded and transported to site on special self-propelled hydraulic trailers and installed over four levels. The largest module weighed 145 tonnes and was 23.6 m by 3.9 m by 2.9 m.

In situations were the available length is even shorter, another option could be to have the flue gas enetering the large casing face, rather than the standard small casing face. This is a feature specific to the vertical HRSG design and allows sufficient space for the inlet duct arrangement underneath. This option can be applied up to middle-sized HRSGs and is useful in repowering projects where the existing steel structure is to be reused.

Modular construction aids construction

The pre-fabricated heat exchanger modules were factory assembled under strict quality control criteria and hydrostatically tested prior to shipment.

As per CMI’s standard design, each module comprised parallel sepentine tubes mounted on support plates and connected to headers at each end. Because of their pre-fabricated design only the final header-to-header joints had to be welded on-site.

The limited site access around the operating unit repowered in the first phase of the project meant that for the second phase it necessary to move the modules into precise final position inside the boiler frame using special hydraulic trailer transporters, which were self-propelled rather than pulled by a tractor.

There was no room on-site for a conventional heavy crane to lift and place the modules in position, so it was carried out by synchronized hydraulic lifting jacks installed at the top of the steel structure.

Self-propelled trailers had to be used to transport the HRSG modules around the congested site at Senoko
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The first module was positioned in the frame and its seven tube support plates attached to the jacks through suspension cables and plates. The hydraulic trailer was then lowered, transferring the full weight of the module to the jacking system. The process was repeated with the other two modules and the entire assembly was raised to allow the next set of modules to be moved into position below the first, the process being repeated until all four sets were suspended. The complete suspended assembly with a total weight of approximately 1450 tonnes was subsequently lifted into position under precise control by the jacking system, with maximum positioning tolerance of between1-2 mm.

Locking pins were then inserted, allowing the full weight to be transferred to the suspension mountings on the boiler frame, thereby releasing the jacks and suspension cables, which were dismantled and reinstalled on the next unit. The high degree of pre-fabrication and modular construction eliminated the need for extensive welding operations on internal pressure parts, other than a few tube-to-tube welds between levels one and two.

The whole operation from offloading at Sembawang harbour to final suspension of the twelve modules onto the boiler frame was completed in six working days, and was carried out without the use of any scaffolding.

Climate dictates Indoor HRSG

Because of the tropical climate and coastal location of the Senoko power plant the operator had specified a fully enclosed indoor-type HRSG. The containment building itself provided acoustic damping and protection against airborne salt, high ambient temperatures and high relative humidity.

Standard boiler stack design was fitted with a flap damper for weather protection and for boiler bottling-up to reduce heat loss during shutdown. To also reduce the overall visual impact, external sidewall cladding was extended 2 m above roof platforms to conceal equipment, such as silencers and natural ventilation louvers.

Repowering makes economic sense

There is real potential for the repowering of old steam turbines in Russia. In many old conventional plants, while the boilers have exhausted their useful life the steam turbines have not.

Repowering those old steam sets into modern combined-cycle with new gas turbines and HRSGs offers the possibility to increase capacity, operating efficiency and flexibility, as well as to reduce emissions.

Today with gas turbines capable of providing sufficient exhaust energy for steam turbines in the 120-150 MW range, there are numerous opportunities for repowering of machines installed in the 1970s, many examples of which can be found in Russia.

The Senoko plant is a good example of a successful repowering project, where the vertical-type HRSG proved to be hihgly accommodating of

It is important to remember that unlike new-build greenfield power projects, repowering demands a high level of customized design and special attention to space limitations, which was typified by the HRSG installation at the Senoko power plant.

Russia land of repowering opportunities

Currently, Russia has in the region of 220 GW of installed capacity, but demand for electricity continues to grow as the country’s economy continues to strengthen. However, the current fleet of power stations is old and will cannot meet the demand. It is estimated that over the ten next year, close to ten cycle-combined plant will be need to be added per annum. CMI Energy has been working in this region for some time, and currently has three on-going projects.

Its latest contract is in Surget, Siberia, where it will be delivering two horizontal boilers with triple pressure, plus reheat for installation behind GE9FA gas turbines to interregional electrical producer, OGK-4, which is a subsidiary of German utility E.ON

The company is also involved in a $570 million project, again for OGK-4, where a new 400 MW unit is being built at the Shatura plant, which is a 1100 MW plant powered by natural gas, coal and peat, and one of the largest in the Moscow region and one of the oldest in Russia. The new unit is due to be commissioned in late 2009.

CMI is also working on the Nevinnomyssk project, and will provide a vertical boiler, triple pressures plus reheat for a Siemens SGT5-4000F turbine. This boiler will be installed at the Black Sea. The customer of the latter project is OGK-5, another interregional electrical producer, which is owned by Enel of Italy.

Both generators have launched vast modernize programmes for their electricity generation fleet, so clearly a Russian energy revolution for both new-build and repowering is on the way.

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