EFFICIENT ENERGY SUPPLY (ELECTRICITY AND DISTRICT HEAT) FOR THE CITY OF LINZ - Johann Gimmelsberger
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Producing more with less: Efficiency in Power Generation
EFFICIENT ENERGY SUPPLY (ELECTRICITY AND
DISTRICT HEAT) FOR THE CITY OF LINZ
Johann Gimmelsberger
Linz Strom GmbHEFFICIENT ENERGY SUPPLY (ELECTRICITY AND
DISTRICT HEAT) FOR THE CITY OF LINZ
Johann Gimmelsberger, Linz Strom GmbH
History
Linz Strom GmbH is a company belonging to the Linz AG. The Linz AG is the leading multi
utilities company in Upper Austria. In 1970 two power station units were put into operation to
form “Linz Mitte – power station”. At this time this was a pioneer in combined heat and
power application for district heating. The fuels utilized were coal and heavy fuel oil.
Environmental protection measures imposed by the City of Linz lead to the addition of flue
gas desulphurisation in 1990. These stringent protection measures imposed on the City of
Linz meant that domestic fuels were substituted by district heating and consequently the
demand rose from 80 MW to 275 MW within 20 years.
A big step forward was gained in 1993 with the start up of power station “Linz Süd”. Where
two GE Frame 6 DLN gas turbines, two heat recovery steam generators (HRSG) and an
extraction condensing turbine were installed.
In 1997 a peak shaving gas turbine GE frame 6 was installed. In 2001 the upgrading of the
open cycle gas turbine with HRSG and the back pressure steam turbine was made
necessary by the liberalisation of the electricity market and the steady growth of district
heating demand.
Modernisation
The premisses behind the modernisation of “Linz Mitte – power station” proved to be very
difficult.
The electricity market became more complex and less predictable. Long term agreements
and capcity prices were replaced by base / peak prices, spot market etc. and the price of
electricity droped due to huge excess capacities in the European power market.
The numerouse discussions about the greenhouse effect, climatic change and fossil fuels did
not in general facilitate the decision process.
The final decision was that the modernised power station ought to achieve high efficiency, a
high power to heat ratio, high fuel utilisation and also promote sustainable development.
District heating systems in general suffer from low full load operating hours and great load
differences during the day and from season to season. The volatility of power prices and the
necessity to produce heating power according to weather conditions and the problem of
predicting both stiffened the requirements.
To overcome these barriers different systems were evaluated: different sized gas turbines
with and without supplementary firing, gas turbines with heat recovery boilers and heat
recovery steam generators, extraction/condensing turbines as opposed to backpressure
turbines, peak load boilers and hot water storage tanks to accumulate energy.
Feasibility studies showed the advantage of the system: a gas turbine with high electric
efficiency, a two pressure heat recovery steam generator, an extraction backpressure steam
turbine with the heating up of district heating water in progressive stages and a hot water
storage tank.
The gas turbine was made by GE, the HRSG came from Alstom Brno and the steam turbine
was provided by Siemens Görlitz. The general contractor was VA Tech Hydro.Hot Water Storage Tank The hot water storage tank measures 65m in height and 26m in diameter with a volume of 34.500 m3 . The tank is operated at atmospheric pressure with a “steam cushion” on top to prevent the ingress of air. The tank is made from boiler plating and was welded on site. 500mm of insulation with a thermal conductivity coefficient of less than 0.05 W/m2K reduces the heat loss to a minimum. The span between the feed temperature (97°C) and the return temperature of district heating water (57°C to 60°C) results in a maximum capacity of approximately 1300 MWh. The physical principle behind the storage tank is quite simple. Hot water is fed in at the top of the tank at very low speed and is withdrawn at the bottom when it is charged and the whole process is reversed when discharged. (see figure 1). The know-how of design and construction is from Dr. Anders Hedbäck (S), who has designed several storage tanks through out the world. VAM Anlagentechnik und Montage were responsible for construction. Operation of the Storage Tank The storage tank is designed for weekly operation. This means, that the tank is charged and discharged within one week depending on the excess heat from power production and the heat demand from the district heating system. In winter the tank is charged during the night (10.00 pm to 6.00 am) and discharged during the day. In summer energy is stored during the week and is used at weekends. (see figure 2) In spring and in autumn, the operation depends on power prices and the heat demand. At these times the most important feature of the accumulator is that the morning district heating peak demand is shaved. No peak load boilers are required. During the day the power production units can be operated at constant load and excess heat is stored in the accumulator. This leads to the nearly autonomous supply of electricity and heat. The benefits of the heat storage tank are the reduced operation of peak load boilers, higher fuel utilisation as compared to condensing extraction turbines, and consequently fuel and CO2 savings. The stored energy can be used as back up energy and contributes to ensuring supply. Biomass Power Plant The conditions behind the biomass power plant are different to those of the combined cycle power plant. First of all the power to heat ratio of the biomass power plant is much smaller. Heat production is the main product and electricity is a kind of a by-product. On the other hand, the production of electricity determines the economic benefit (promotion of green / renewable electricity). The fuel market (wood, residues from the timber industry and from forestry) is not as developed as the fossil fuel market. The building of a big power plant could affect the market in a way that is not beneficial to the project. The transport of biomass is more complex than natural gas and has an effect on both ecology and economy. The specific price of the technology required for the generation of electricity is quite high when compared to that of combined cycle technology.
Our objective was to design a biomass power plant with maximum fuel utilisation, high
electricity output with innovative and reliable technology.
Therefore the significant dimension behind the design was the load duration curve of the
district heating system.
The technology used is a biomass fired Rankine cycle with a backpressure extraction
turbine. The steam extracted is used internally in the power plant. The exhaust steam is
condensed at 0.8 bara to provide district heating water with a temperature of 80°C and high
electricity output.
The Data
Thermal Input 35 MW
Electrical Output 8 MW
District Heating Output 22 MW
Technology
The fuel is fed into the boiler on a wandering grate via a spread stoker. There is high
turbulency on the grate caused by blowing half the combustion air through the grate. The
high turbulence encourages very effective combustion, so that the air to fuel ratio can be very
low. The secondary air and recirculated flue gas is blown in at the front and at the back of the
combustion chamber. The combustion chamber consists of finned walls like the flue gas
passes. The boiler is equipped with 3 superheaters and economizers for combustion air and
feedwater.
Aalborg Energie Technik (DK) is responsible for the engineering and construction of the
power plant.
The fuel biomass will provide 15 to 17% of the heating energy of the district heating system
of Linz. This is a significant contribution to achieving Upper Austria´s targets in renewable
energy utilisation. For Linz Strom the biomass contributes towards securing supply and fuel
diversity.
Conclusion
To achieve a sustainable energy supply it is obligatory to invest in up-to-date technology. But
this is not sufficient. The technology used has to perfectly fit into the energy system.
Clean production, high efficiency (in a technical as well as an economical sense) and
security of supply are key-factors in this business. We are convinced that we can meet these
requirements with our modernized power station “Linz Mitte”:
• Combined cycle power unit
• Biomass fired power unit
• Heat accumulator
The modernized power plant is designed for maximum fuel utilisation. This causes
constraints but flexibility is regained by the heat accumulator. So what we actualy operate is
an electricity orientated heat-focused set up.80-130 District heating
°C
Heat storage tank system flow
(„hot“)
Heat-
Supplemen
generator
tary heating
(> 97 °C)
97 °C
- charge
- discharge
Heat-
generator
60 °C 60 °C District heating
system return
(„cold“)
Figure 1Thermal capacity during a typical week Thermal capacity during a typical week
in winter in summer
1400 1400
1200 1200
1000 1000
800 800
[MWh]
[MWh]
600 600
400 400
200 200
0 0
0 24 48 72 96 120 144 168 0 24 48 72 96 120 144 168
Figure 2You can also read
