The Benson* once-through HRSG can help cut combined cycle plant start-up times dramatically. The technology is now being offered commercially.
At the heart of the fast-start combined cycle power plant technology recently rolled out by Siemens Westinghouse and now being offered commercially is the Benson once-through HRSG.1 This, together with a high-capacity steam bypass system, enables the gas turbines to be started as quickly as in a simple cycle mode. In addition, the use of a highly integrated system design, including advanced water treatment systems, upgraded automation, auxiliary boilers, as well as a patented Turbine Stress Controller (TSC) results in quicker start-up of the steam portion of the plant. As a result, plant start-up is as much as twice or even three times as fast as a typical 2-on-1 F-class combined cycle plant after overnight and weekend shutdowns. The resulting lower fuel and water consumption and lower start-up emissions are added benefits.
Siemens estimates that plant operators can potentially achieve savings of about $24 million (Net Present Value for 20 years) in fuel alone, as well as a substantial reduction in NOx, CO and VOC emissions and in water consumption compared with existing plants.
Since the fast-start plant can be operated equally as well, without adverse effects on efficiency, in a spot-peak load, intermediate-load or a base-load mode, there is no need on the owner’s part to try to predict the operating requirements throughout the life of the plant. The benefit of such flexibility is considerable. Furthermore, the technology makes it possible for plant operators to respond quickly to changing market demands and maximise potential power generation revenues.
The Benson once-through HRSG fast-start technology was developed in the 1990s and culminated in the successful application at a 390 MW combined cycle plant at the Cottam Development Centre in Nottinghamshire, UK (originally a Siemens/Powergen joint venture but now wholly owned by Powergen, a subsidiary of E.On). This single-shaft facility, which has been in commercial operation since 1999, is started on a daily basis and is load cycled to respond to grid demands.
The 2.W501F Fast Start plant
From a plant operating capability standpoint the change of power plant operational regime towards more intermediate duty significantly compromises economics, particularly when the asset is designed as a base-load facility. The resulting “hidden” or unknown costs of cycling include those arising from increased forced outages, lost production, increased frequency of failures in HRSG and steam turbine components.
The major challenges facing asset managers in an intermediate-duty market are responding quickly to dispatch demands during high-profit opportunities and dispatching while revenue generation is feasible. The majority of combined cycle plants do not fully support these challenges and little progress has been made in addressing them.
In today’s market conditions it is assumed that combined-cycle power plants require the operational flexibility that allows them to operate in intermediate duty with the same efficiency as that of a base-load plant. It was found that the long established differentiation between hot, warm and cold starts with the corresponding downtime assumptions of 8 h (hot), 48 h (warm) do not really reflect current operating scenarios.
The 2.W501F Fast Start plant, developed by Siemens Westinghouse, is a 2-on-1 multi-shaft combined cycle plant (using two W501F gas turbines) that aims to respond to these problems. It assumes the following operating regime:
Downtime Starts per year
Overnight 8-16h 200
Weekend Up to 64h 50
Extended >64h 2
To increase the plant operational flexibility eight additional extended shutdowns were taken into consideration for component lifetime evaluation. While the plant is designed to accommodate some 260 starts/4000 operating hours per year it preserves its baseload operating capabilities.
Under the umbrella of the Siemens Reference Power Plant programme, component and system designs were brought together to achieve the following goals:
• The ability to ramp up the gas turbines in combined cycle applications as quickly as in simple cycle, unrestricted by downstream components including the HRSG and steam turbine.
• The ability to vary and quantify effect of start-ups on the steam turbine maintenance schedule, enabling the asset manager to set a business strategy considering the ST and HRSG, along with GT maintenance.
The diagram top right compares the relative power output dispatched by the 2.W501F Fast Start plant and a typical plant, following a 16 h overnight shutdown (both assume three-pressure reheat bottoming cycles).
The fast-start plant start begins with concurrent ignition of both gas turbines (1); the gas turbine generators are synchronised at (2). After an unrestricted GT ramp-up the GTs reach rated power at (3). At this point the steam turbine is also rolled off. The ST generator synchronises at (4) while the hot reheat and low-pressure system bypass valves are closed at (5) and the ST accepts all generated steam. At this point the plant reaches approx. 97% of its rated power output.
This optimised scheme allows the 2.W501F Fast Start plant to start up about twice as
quickly as its predecessor after overnight and weekend shutdowns.
The fast-start technology is also being applied to other combined cycle configurations, including 1S.W501G, 2.V94.3A (as at Cottam) and 1S.V94.3A.
Key to this improvement is the use of the Benson Once-Through steam generator, which can be applied with and without supplementary firing and SCR/CO-catalyst systems.
Benson OTSG
The Benson technology has proven its capability and reliability throughout the past 70 years. This technology is well known throughout the industry in both subcritical and supercritical designs in conventional steam power plants. To-date about 1000 Benson boilers with a cumulative steam output of more than 700 000 tons per hour have been built.
In the 390 MW combined cycle application at Cottam, the Benson OTSG has shown its direct applicability for fast-start up and cycling operation, even though the Cottam plant was not explicitly designed for this duty.
Based on US market demand for fast start-up/shutdown and cycling capabilities and the successful testing and operating experience at Cottam, the Benson-OTSG was incorporated in the 2.W501F Fast Start plant.
In the Benson OTSG, the evaporator stage is characterised by two bundles of vertical tubes, which are arranged in series in the horizontal gas path. All tubes within an evaporator bundle are connected in parallel, creating natural circulation characteristics. The low mass flux design allows the fluid mass flow to self-adjust to the heat input in each tube row, meaning tubes with higher heat absorption will see an increased fluid mass flow. This design ensures static and dynamic flow stability throughout a wide load range. In addition it results in lower pressure losses across the evaporator compared with other once-through concepts.
The Benson-OTSG is capable of handling high temperature transients during a fast start-up due to the elimination of thick-walled steam drums. The HP drum, in particular, impedes fast start-up of conventional HRSGs by dictating long soaking periods and slow maximum allowable temperature ramp rates. The Benson-OTSG eliminates these restrictions by replacing the drum with small separator vessels, which also provides a more economical arrangement. The separators perform the function of water/steam separation during start-up and shutdown. At steady state operation, including low loads, the steam flow passes through the separator, as part of the interconnecting piping toward the superheater. The steam at the evaporator outlet is slightly superheated; consequently no separation occurs.
Another common feature of once-through technology is utilised to control the main steam temperature. In contrast with a drum-type HRSG, the Benson-once-through HRSG is able to control the main steam temperature by modulating the feedwater massflow. This enables the system to compensate for the effects of changing ambient conditions or GT loads within a certain range without placing the steam attemperators in operation. Attemperators are incorporated as interstage, as well as final-stage desuperheater for the high pressure and reheat steam systems. This arrangement ensures compliance with the steam temperature limits, dictated by the steam turbine, during plant start-up.
Simply stated, the Benson once-through steam generator retains all the positive features offered by the traditional drum-type HRSG, including provisions for SCR, CO catalyst, duct firing, steam power augmentation, etc, while providing additional operating flexibility over the drum-type HRSGs.
Steam turbine
The 2.W501F Fast Start plant is equipped with a KN-type steam turbine. The steam turbine is started power-output oriented, which means that it is loaded fast with reduced and nearly constant steam temperatures until the bypass valves are closed with the gas turbines at base load. This also eliminates steam vents to atmosphere at the HRSG, due to the incorporation of an auxiliary boiler. Since there are no HRSG or ST induced holds of the GT start-up process, special care must be taken to handle the generated steam in order to meet the optimal steam temperature for fast loading of the ST.
The K-type steam turbine is designed for full-arc admission without the use of a control stage. This design greatly reduces blade stresses during the start-up process. The combination of full-arc admission with variable-pressure operation (which has become an industry standard for HRSGs) provides high efficiency, maximum operating flexibility and minimum maintenance.
The application of high-capacity steam bypass systems for HP, IP and LP steam is key to the operating flexibility and reliability/availability of any plant. The 2.W501F Fast Start plant incorporates provisions, long proven in European power stations, to route the HP bypassed steam into the cold-reheat of the HRSG to minimise thermal stressing of the HRSG reheater section during start-up.
Further flexibility comes from the use of the Turbine Stress Controller. This consists of a stress evaluation system based on a steam turbine start-up program with an on-line life cycle counter. It calculates the stresses in all critical ST “thick wall” components (including the valves, HP casing and rotor body and the IP rotor body) depending on three different ramp rates, normal, accelerated and fast. The function of the TSC is to control the start-up ramp rates so as to minimise material fatigue within the constraints of operating requirements and at the same time to calculate the cumulative fatigue of the monitored turbine parts. The TSC assigns an appropriate number of equivalent starts for each normal, accelerated and fast start. With this knowledge of the effect of each type of start on the maintenance schedule of the steam turbine, the owner can make prudent business decisions regarding whether or not the asset should be used to satisfy transient dispatch requirements.
Steam/water cycle and chemistry
While the power market requirements have changed and many combined-cycle plants are being operated between peaking and intermediate duty with demand for fast start-up/shutdown and cycling capability, asset owners still expect base-load plant efficiency levels. This is achieved not only by retaining the triple-pressure reheat steam/water cycle, but also by integration of the GT and the cycle to maximise efficiency through systems such as rotor air cooling, fuel gas heating and steam cooling of the GT transitions (for the W501G).
This cycle design has been thoroughly analysed and as previously mentioned was first introduced at the Cottam Development Centre. The Benson-OTSG is a once-through steam generator for the HP and IP part, while the LP section retains a drum-type cycle. This HRSG configuration requires a combination of pH-control with ammonia and oxygenated treatment. The oxygenated treatment creates and maintains a protective layer of magnetite and haematite inside the tubes.
As a further enhancement to rapidly achieve proper steam quality and to support fast start-up, the cycle incorporates a condensate polisher. The polisher provides high quality feedwater to the Benson-OTSG thus avoiding deposits in the evaporator tubes and thereby decreasing the evaporator pressure losses. The condensate is deaerated in the condenser hotwell by vacuum, while the polisher removes the ammonia, which must then be added to the polisher effluent for proper pH adjustment.
The combined oxygen/ammonia treatment with integrated condensate polisher, as incorporated in the 2.W501F Fast Start plant design, has already contributed to the high reliability of Cottam Development Centre and reduced the time to meet steam purity requirements to minutes.
Other noteworthy measures applied in the 2.W501F Fast Start plant to support operating flexibility include components and systems that have been long proven in European plants, such as: component redundancies; auxiliary boiler; vacuum pumps; stack damper; automated drain and vent valves; optimised steam piping warm-up concept; and a high level of system and plant automation.
Other benefits
Regarding emissions, the only certainty for the future is that the caps imposed by the regulating agencies will become more stringent. This means that importance of start-up emissions will also increase. The 2.W501F Fast Start plant provides a combined reduction of NOx, CO and VOC emissions of about 150t/year when run in the kind of scenario described above. The additional benefit, which is difficult to quantify, is that without these emission reductions, plant operation could be restricted or curtailed, eg, in non-attainment areas.
Other benefits, as already noted, are that, due to accelerated gas turbine start up, the 2.W501F Fast Start plant has significantly reduced fuel requirements, and since the fast-start plant eliminates start-up vents, there is also a saving in water consumption of about 6 million gal/year (about 63 000 m3/year).
Overall, the fast-start capabilities have a positive influence not only on plant economics and operational flexibility but also on environmental impact due to enhanced part load capability, without sacrificing high efficiency for baseload operation.
For further information contact
Raymond Baumgartner,
Siemens Westinghouse Power Corporation, Orlando, Florida, USA,
tel +1 407 736 3010,
raymond.baumgartner@siemens.com
Alstom also opts for Benson OT technology
Alstom has recently announced that it is incorporating the Benson once through evaporator technology developed by rival Siemens into its “next generation” heat recovery steam generator technology, where it will be married with Alstom’s own OCC (Optimised for Cycling and Constructibility) concept (see Modern Power Systems, July 2001).
Alstom notes that a key advantage of a once-through evaporator in an HRSG is the elimination of the thermally sluggish, high-pressure steam drum. But the Siemens technology (with horizontal gas flow, vertical tube arrangement for the evaporator, as described in the main article), in addition to the elimination of the steam drum, also provides improved cost effectiveness and the ability to provide a once-through HRSG in standardised heat exchange modules.
OCC includes the following measures to reduce thermal stresses during cycling: single row of tubes between headers; finned tubes with no bends; flexible connections between pressure part sections; elimination of division walls inside headers; high creep strength materials in high temperature area; small diameter headers; multiple header connections to promote uniform flow and metal temperatures in superheater and reheater sections; and enhanced drain arrangement to prevent condensate flooding of superheat and reheat sections during gas turbine purge.
In addition, the once-through OCC design can be packaged and delivered with different degrees of modularisation:
• Harp bundles – where transportation is restricted, large cranes are scarce and site labour is inexpensive;
• Pressure part modules – where transportation is less restricted and large cranes are available; and
• C-frame modules (pressure parts integrated with support steel and casing) – where transportation is unrestricted, large cranes are available and site labour is expensive.