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Reheat steam turbines for sustainable energy solutions

21st February 2013


In today’s power generation market, the demand for power is undergoing a transformation. Higher oil and gas prices and awareness of greenhouse effects are contributing to an increasing demand for renewable energy in the portfolio of many utilities.

One way to contribute to the sustainability of the environment and its energy resources is in the implementation of industrial steam turbine solutions for alternative energy applications.

For waste-to-energy, solar power and biomass, an impressive number of proven references already illustrate the capacity of these steam turbine solutions.

Waste-to-energy (WtoE) is a process by which forestry and agricultural waste (biomass) and municipal solid waste (garbage) is incinerated at high temperatures to produce steam, which then passes under high pressure through a steam turbine to create electricity.

Because the cost of the fuel in waste-to-energy facilities is virtually non-existent (in most cases revenue can be generated from disposal of the waste) it represents a viable energy alternative to fossil fuels which are non-renewable and cost money.

Siemens provides industrial steam turbines which fit perfectly into this increasingly viable and popular application.

The most powerful solution for such waste-to-energy plants is the high efficiency reheat

SST-700RH steam turbine which consists of one geared high-pressure (HP) module and one direct-drive low-pressure (LP) module.

The SST-700 has a high volume flow exhaust area to achieve maximal performance: with its exhaust area of up to 9.6metres, this is the biggest industrial steam turbine Siemens supplies. Proven standardisation ensures low investment and life cycle costs.

How does the reheat process work? An outline follows:

  • A heat recovery steam generator after the boiler provides superheated steam at high temperature and high pressure. The exact parameters vary, depending on the type of plant in which the process is used.
  • The steam is admitted into the HP-turbine. In the turbine there are several stages (a row of stationary blades + a row of rotating blades) where the steam will ‘expand’ as the steam pressure reduces after each stage. First the steam increases the speed in the stationary blade and then the high velocity steam enters the rotating blades and forces the rotor to move.
  • From the HP-turbine exhaust the steam is taken back into the steam generator for re-heating to raise the low pressure steam back to the original temperature.
  • The re-heated steam is now admitted into the LP-turbine to generate further power in a set of stages, finally entering into a vacuum condenser where the remaining steam is condensed. The resulting water is pumped back into the steam generator to generate the steam used in the closed loop process.
  • The two turbine modules are connected to an electrical generator providing power to consumers via the grid.

The SST-700 reheat solution was selected by Amsterdam’s Waste & Energy Company for its 4th generation WFPP (Waste Fired Power Plant) in 2003.

With the SST-700RH at the heart of the process, the plant was designed to burn 900000tonnes of waste per year at an efficiency of 22percent. During 2007, this will have increased to 1.5milliontonnes at an efficiency of 30percent.

Solar power

Today we are witnessing the start of a new era for concentrated solar power plant (CSPP). In North America, new plants are currently under commissioning, the first since the 1980s. In Europe, the Spanish government has granted a price surplus for solar produced power valid for 25 years. Given the increasing demand for clean and environmentally friendly power, solar power has the potential to expand rapidly if it is both reliable and competitive.

The high investment cost for a CSPP with limited operational hours necessitates a high efficiency perspective right from the beginning and this imposes high demands on the steam turbine used in the process. In close cooperation therefore with Solargenix (Boulder City, USA) and Sener/Cobra (Andasol, Spain), Siemens have developed and fine-tuned the SST-700RH steam turbine, now optimised also for solar steam cycles and capable of generating up to 175MW.

The highly efficient SST-700RH with its high-speed, high-pressure module enables a smaller solar mirror collector field; in addition, the surplus heat can be stored in large storage tanks and used to extend the running hours of the steam turbine. Flexible extractions for preheaters permit an optimised cycle design.

The reheat process is essentially the same for a CSPP as for a WtoE plant, but the daily cycling and low temperature in a CSPP plant require special attention. The SST-700RH uses high quality materials especially chosen for long and trouble-free operation in a solar plant. When focusing on annual power production, short startup and stop times are of great benefit.

The SST-700RH, with its low mass rotors and casings, is ideal for daily cycling and has a low minimum load, enabling maximum running hours per day for plants without heat storage. The cycle has also been optimised for nocturnal standstill.

With the right technology now available, solar power is becoming a strong alternative for the growing demand for clean and reliable power.

The setup of the steam turbine and the preheater configuration used in Boulder City is a geared HP turbine with direct-drive LP turbine.

The cycle consists of four preheaters (one HP preheater, one deaerator, and two LP preheaters) with one bleed from the HP turbine exhaust and three from the LP turbine. The CSPP in Boulder City has a nominal plant output of 64 MWe and a nominal steam turbine output of 72MWe.

Biomass

In environmental terms, biomass is an extremely valuable source of energy, being defined as a CO2-neutral fuel alternative and a renewable energy source.

Since available resources are limited, maximum utilisation of the fuel is very important and the technologies to be applied have to be efficient, reliable and cost-effective.

Reheating is proven as an effective means of improving plant efficiency, by as much as 5percentage points. In addition, as electricity tariffs and current prices of biomass fuel improve economic feasibility, the industrial steam turbine reheat solution will increase the profitability of the overall plant investment.

The SST-PAC 400 reheat turboset, (PAC indicates that the SST-400 turbine is combined with a generator or other driven equipment) is the most popular Siemens solution for biomass applications. It consists of geared high-pressure (HP) and low-pressure (LP) turbines from the standard range with the same speed and one gear for both.

Each turbine can be optimised for its specific requirements. The HP turbine is optimised for a small volume flow of high-pressure, high-temperature steam, while the LP turbine is optimised for the large steam flow volumes at a lower steam pressure. The gear unit is located between the LP turbine and generator.

Helped by local legislation, Siemens has many references in Germany and Austria for this application, the most prominent being Simmering in Vienna, and Koenigs Wusterhausen, south of Berlin in Germany. With an electrical efficiency of over 35percent, the German plant is equipped with highly efficient flue-gas treatment systems and will save 120000 tons of carbon dioxide emissions annually.

Conclusion

Energy consumption is increasing at a phenomenal rate, and energy resources will not last for ever. Pursuing alternatives to conventional power production is essential if we are to meet our energy and environmental targets. Whether we are talking in terms of total cost of ownership for plant efficiency and economy, energy mix for security of supply, or decentralised supply for covering peak power demands, sustainable and flexible energy sources are the way to go.

Enter 36 or at www.engineerlive.com/ipe

Lynne Anderson is with Siemens Power Generation, Petaling Jaya (Kuala Lumpur), Malaysia. www.siemens.com/powergeneration 

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