Co-Firing Projects Case Studies
Avedøre power station is located in Copenhagen. It is one of the world’s most energy-efficient combined heat and power plants, operated by DONG Energy. Avedøre power station produces electricity to the Nordic power grid and district heat to the Copenhagen metropolitan area. The heat produced in Avedøre accounts for the needs of 280,000 single-family houses. Coal and oil-fired Avedøre unit 1 started operation in 1990 and multifuelled Avedøre unit 2 in 2001. Avedøre 2 is divided into three modules: an ultra supercritical boiler plant, a gas turbine plant and a biomass plant. Avedøre 2 uses a new multifuel concept and embodies a range of cutting-edge technologies, including an ultra supercritical boiler, the world’s most advanced steam turbines, the largest straw-fired biomass boiler yet build and areoderivative gas turbines for feedwater preheating.
The 570MW Avedøre-2 power plant can work from biomass, and incorporates other measures to reduce environmental impacts. The Danish Energy Agency approved E2's plans for the new plant at Avedøre, south of Copenhagen in September 1994. It was jointly developed between Energi E2 and Vattenfall. The plant is located adjacent to an existing coal fired 250MW CHP plant, Avedøre-1, and was required to make minimum impact on the environment. As a condition of the project approval, SK Power had to decommission three older coal-fired power plants to reduce net emissions of CO2 (10%), NOx (20%) and SO2 (30%).
In the BioCoComb demonstration project, supported by the EC Thermie programme, a CFB gasifier for bark, wood chips, sawdust etc. was installed at the 137 MWe pulverised coal fired power plant of Verbund Austrian Hydropower AG in Zeltweg, Austria. The project team was formed by Verbund/Draukraft (Austria), ENEL (Italy), ESB (Ireland), EVS (Germany), Electrabel (Belgium) and Austrian Energy and Environment as the supplier of the gasifier. Scientific advise was given by the Technical University of Graz (Austria).
The process concept is based on the gasification of biomass (bark, wood chips, sawdust with a water content of 40-50%) in a fluidised bed. In this case, the air is fed to the system to exactly such an extent that part of the fuel burns and, while doing so, the heat is produced that is required for the gasification of the rest of the biomass, for the combustion of which not enough oxygen is available.
Because it is neither a matter of total combustion nor a matter of total gasification it is called partial gasification. The gas is led uncooled from the gasifier to the boiler, where it serves as auxiliary fuel and replaces part of the coal. Apart from the CO2-reduction, the NOx-reduction through „reburning“ is also of interest.
For integration into the power plant the fluidised bed gasifier is installed near the coal-fired boiler. In the gasifier the biomass is converted to gas that is then directly conveyed to the boiler via a hot-gasline as a second fuel. The partial gasification taking place in the reactor is sufficient respectively desired. Due to this process, pre-drying of the biomass and cleaning of the emerging gas is not necessary. Furthermore, this process can be used to reduce NOx-emissions, due to the fact that, with the aid of the gas, a second combustion takes place in the coal
The “Tekniska Verken Ltd” is a company owned by the Linköping municipality some 200 km south-west of Stockholm. Linköping is the 5th largest city of Sweden with 130 000 inhabitants. Tekniska Verken Ltd supplies 90,000 customers with 4 PJ district heating and 4 PJ electricity. The CHP plant annually produces about 150 GJ of heat and 60 GJ of electricity.
The first part of the plant was built in 1964. It comprised two boilers using oil as fuel supplied by “Svenska Maskinverken” from Gothenburg. In the seventies another boiler and a new steam turbine were installed and one of the old turbines from 1964 was also changed to a larger one. In 1984, due to the increased oil prices on the world market, the two original boilers from 1964 were redesigned to utilise solid fuels. One of the boilers was converted to use coal. The boiler was equipped with a Spreader-stoker with a wander grate supplied by Vølund from Denmark. The second boiler was converted to burn biofuels on a moving grate and was supplied by Von Roll from Switzerland. Today, the boilers are integrated in a CHP block: one for coal, one for oil and one for wood-fuels with an total capacity of heat production 240 MW and electricity in CHP of 77 MW.
The fuel mix is chosen in respect of fuel prices and the national taxation system. The operating time is from September to May with planned revisions and maintenance during the summer.
Stora Enso Fors Ltd is a card board mill located in Fors, 150 km northwest for Stockholm, Sweden. The plant is owned by the company Stora Enso Ltd, which is a Swedish/Finnish owned company, one of the largest in the pulp and paper industry. Paper products have been produced at the mill since 1950.
The company owns an integrated combined heat and power plant (CHP) of the paper mill and supplies the mill with process steam and electricity. There are four different boilers within the mill: one circulating fluidised bed (CFB) boiler using biofuel and coal, two oil fired boilers at 12.5 MW and 65 MW and an electric boiler at 20 MW. The main CFB boiler, studied in this project, was delivered in 1985 by Ahlstrom (currently owned by Foster Wheeler) with the following data for combined heat and power production:
- Boiler capacity 55 MWth electrical output 9.6 MWe (back-pressure steam turbine).
- Steam data: 20 kg/s, 60 bar, 475° C.
- Process steam taken off at both 12 bar and 4.5 bar.
- Bed area: 275 m2
Each year, 6000-7000t (147 – 171.5 TJ) of coal are used. It has an average net heating value of 24.5 GJ/t. Around 150,000t/year (1230 TJ) of biofuel are used with an net energy value, LHV of 8.2 GJ/t. The biofuel is burned in various forms including wood chips, bark and sawdust and rejects from the card board production. The average moisture content in the biomass is 46% and 10% in the coal.
The CFB boiler produces steam for both the production of electricity and for the plant’s process steam requirements. It is possible to burn a wide range of fuels in the boiler including coal, biofuel, oil and waste materials such as paper, paper board and plastics. The combustion temperature within the boiler is controlled to between 810°C and 930°C. When running at full load (55MWth), 26t/h (6-8 kg/s) of biofuel is being fed into the furnace. Coal is currently used only as a support fuel when the biofuel heat value is too low to match energy demand. Normal temperature in the furnace is 850°C and in the top of the boiler before the cyclones about 900°C.
The flue-gas temperature after the economiser and air preheater of the boiler is at 150°C giving a total efficiency of 89%.