In the jewel of Austria, Vienna, power demand is steadily rising at between two and three per cent annually and demand for heat is also at the upper end of this range. As a result, capacity margins began to narrow and, as prices approached €20-30/MWh, utilities started to consider new power developments in order to meet demand and capitalize on larger margins.
Faced with such a situation Wien Energie, a major utility serving the Vienna region, launched a programme to increase its generation capacity. Its two-unit gas fired plant at Simmering, just outside Vienna, which was originally put into operation in 1978 was long overdue for replacement, given a typical lifespan of some 200 000 hours of operations.
Initial plans envisaged developing a complete new facility at the Simmering site, but this was not only deemed an expensive option but also would struggle to gain regulatory approval. Although high efficiency combined-cycle gas fired stations, which utilize combined heat and power technology are one of the most efficient, and therefore economic methods of generating energy, public perception regarding new build plant is a significant hurdle in much of Western Europe.
Vienna, in particular, has a notoriously difficult and tortuous planning regime. Compounding this factor, similar difficulties associated with siting new transmission lines curtail the possibilities for power imports to the Austrian capital. Therefore as a method of both controlling development costs and securing the relevant regulatory approvals, a repowering concept was considered that would effectively boost capacity without the need for a new power plant.
The repowering concept envisaged effectively doubling installed capacity at the site without apparently building a new power station within the city limits, almost like a conjuring trick. Furthermore, by utilizing almost the entire existing distribution infrastructure, including those for gas, ammonia and cooling water entering the plant and for the district heat and electricity that is distributed to Vienna in turn, development costs are significantly reduced. Indeed, the development of two new standard plants would cost around 30 per cent more than repowering, which is designed to achieve a similar improvement in the economics of existing power assets, while reducing both emissions and operations and maintenance costs.
The project for repowering Unit One was initiated in May 2004, with the contract subsequently awarded to Siemens in July of the following year after a Europe-wide public tender process. The power island will use Siemens’ SGT5 – 4000F single-shaft gas turbines coupled to the company’s two pole, air-cooled 1000 A generators. In addition, new heat recovery steam generators (HRSGs), of a three stage vertical configuration with top mounted stacks, are to be installed for each turbine.
The use of existing plant infrastructure includes the steam turbine, condensers, generator, switchyards and transmission lines along with water, gas, and ammonia feed lines. In addition, retaining the existing Unit Two at the site allows the continued use of the 65 MW gas fired plant for peaking heat and power capacity.
Replacement of the existing gas fired steam generator at Unit One with two 260 MW gas turbines, each with its own HRSG, which are subsequently connected to the existing steam turbine, will roughly double net electrical output in full condensing operations from 430 MW to 820 MW. Under full district heating demand, electrical output increases from 360 MW to 700 MW. Thermal capacity, meanwhile, increases from 280 MWth to 450 MWth.
The three-stage steam turbine features high, intermediate and low pressure stages, and uses an intermediate take-off configuration to supply energy to the local heat distribution network. The repowering project will increase electrical efficiency by almost 15 per cent from 42.6 per cent to 57 per cent, and improve overall thermal efficiency to more than 80 per cent when operating at full district heating load and maximum power generation, compared with some 62 per cent previously.
Boosting efficiency to among the highest in Europe also allows net reductions in CO2 and NOx emissions, despite increased capacity. The plant is to be controlled using a Microsoft Windows XP-based Teleperm system allowing just six operators to control the entire site together with around ten or 15 maintenance staff on site.
Siemens’ standardized repowering concept provides several alternatives for integrating a gas turbine into an existing steam plant. In many applications the design lifespan of the steam turbine has been reached, requiring modernization taking the modified steam flow into consideration. A detailed assessment of the existing plant determines the optimal approach to repowering.
Full repowering, as at Simmering, is a complete replacement of the original boiler with a combination of one or more gas turbines and HRSGs, and is widely used with very old plants with boilers at the end of their lifetime. It is considered one of the simplest ways of repowering an existing plant. Repowering can also include modernization of the steam turbine and instrumentation and control systems.
In parallel repowering, the boiler remains in operation for peak and intermediate load. The steam from the existing boiler is added to the steam from the HRSGs at several pressure levels, depending on the condition and capability of the existing steam turbine.
Topping, also known as hot windbox repowering, involves the conversion of a straight steam cycle to a full fired combined-cycle, in which the gas turbine assumes the role of a forced draft fan in a conventional steam boiler-turbine unit. The exhaust from the gas turbine is used as preheated vitiated air to burn the main fuel in the fired boiler.
Boosting is a limited form of parallel repowering. The HRSG added to the plant is not designed to raise superheated steam for the existing steam turbine, but only to preheat the condensate and/or feedwater flows to the associated boiler.
Steam turbine output
For full repowering, the available HRSG steam defines the achievable steam turbine output. As a rule of thumb, the thermal cycle of the repowered unit is targeted to achieve around 70-80 per cent of the rated steam turbine output in order to achieve a reasonable amount of steam flow through the high-pressure section of the steam turbine. In combined-cycle applications, as well as for full repowering, the steam flow through the steam turbine is increased due to the added steam from the intermediate-pressure and/or low-pressure HRSG, and the fact that steam extractions for condensate feedwater are no longer required. Repowering therefore requires detailed assessment of the low-pressure turbine and the condenser’s ability to cope with the increase flow.
For parallel repowering, the steam turbine capacity is selected arbitrarily. The steam flow characteristics enable full utilization of the steam turbine. The combined-cycle mode of operation corresponds to full repowering. In the hybrid mode the existing boiler supplies additional steam up to the capacity limit of the steam duct firing upstream of the HRSG.
The attractiveness of parallel repowering lies in the huge load reserve available at an efficiency that is far higher than for simple-cycle gas turbine or steam plants.
Repower or not?
Typically, an old steam plant has an efficiency of around 37-39 per cent, compared to a new combined-cycle power plant with an efficiency of around 52-58 per cent, depending on the gas turbine and the water/steam cycle configuration. Repowering using existing steam turbine and plant equipment that is not optimized to the new process, leaves the achievable efficiency slightly lower than that of a new combined-cycle plant. Nonetheless, an aging existing gas fired steam plant, which is operated at an efficiency of 38 per cent might raise efficiency to about 55 per cent through full repowering. Besides the performance increase, there is a direct impact on emissions, with CO2 intensity typically cut by around 30 per cent.
According to Siemens, repowering currently accounts for perhaps ten per cent of the European market for its combined-cycle gas turbine installations, although a large proportion of Eastern European plant is aging coal fired and gas fired steam capacity that would be well suited to repowering. With market reforms pressing competition on former monopolies, repowering not only provides an opportunity to improve competitiveness by extending the lifespan of existing facilities and improving performance, but also reduces operations and maintenance costs.
However, despite the very effective reduction in carbon emissions, repowering projects within Western Europe do not currently qualify for carbon certificates. A decision on the issue is expected this year which, if favourable, would significantly improve the economic case for what is already a commercially viable strategy.
For Wien Energie, repowering Unit One of the Simmering plant forms a significant part of its plan to increase its market share. By increasing capacity through repowering, the company has managed to increase its output, reduce emissions and cut operations and maintenance costs at a stroke, and at a cost around one-third less than a new build project. Using existing infrastructure not only cut costs but also improves reliability in a relatively short period of time.
In addition, the recently submitted Austrian National Allocation Plan allows the company to fully use the existing carbon emissions credits awarded to the older Simmering facility and as a further incentive to improve overall environmental performance. In Austria more efficient generation, such as gas fired plants, receive more carbon credits than older and less efficient plants such as those burning coal.
Ultimately, despite an active and engaged environmental lobby, not a single complaint has been lodged regarding the repowering project at Simmering. And after all, why should anyone complain about a reduction in emissions?