As fuel, the plant uses combustible source-separated waste that cannot otherwise be utilised.
Most of the fuel is common household waste. In addition, different kinds of industrial waste, for example, waste from food and wood processing industries, expired groceries and agricultural waste can be utilised in the WtE plant.
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Waste trucks drive into the tipping hall and unload into the waste bunker. To prevent spreading odour nuisances, the doors of the hall are kept closed and they open automatically when a waste truck approaches. The primary air to the combustion is taken from the tipping hall to the grate, and the hall is under-pressured. There is room for five conventional waste trucks or for one sidetipper in the tipping hall.
The waste bunker is 14 meters deep. In a normal situation, there is enough waste for 7 - 10 days in the bunker which equals 3 400 - 4 800 tons of waste. A three weeks’ supply is the maximum amount of waste that can be stored in the bunker. Automatic grabs mix the waste to maintain homogeneity. There is also a crusher in the bunker which is used to crush large items, for instance, furniture.
The area of the grate is 80 m2
. The ram feeder is hydraulically operated and pushes the waste from the feed chute to the first section of the grate where the waste dries. Next, the waste moves downward to the other sections of the grate: pyrolysis, gasification and incineration sections. Hot bottom slag is cooled with water in a slag extractor. The supplier of the grate, bottom slag outlet systems and auxiliary equipment is Hitachi Zosen Inova AG.
Heat exchange from the flue gases to the boiler water takes place mainly in the boiler. In the first stage, the water is preheated. The walls of the boiler consist of pipes covered Inconel coating. The water inside the pipes is heated to steam and then super-heated to 400 degrees and 40 bars in the super-heater section of the horizontal pass after which the hot and high-pressure steam is led to the turbine.
Flue gas treatment
After the boiler, the flue gases are led to a cooling tower and LAB-loop reactor where chemicals that adhere to impurities are added to the flue gases. In the fabric filter, the activated carbon-lime dust and adhered impurities (flue gas treatment residue) is then filtered out. The filter consists of over 1500 bag filters that are made of Teflon textile. The diameter of one bag filter is 13 cm and length 6 m. The total area of the filter is 3700 m2
. The flue gas treatment system has been supplied by a French company, LAB.
After the fabric filter, the purified flue gases are led to the stack via a silencer. The stack is 75 meters high. The composition of the flue gases is analysed and monitored continuously. This is done to make sure that no substances harmful to the environment, for instance, heavy metals, are released. Significantly, the emissions of a waste-to-energy plant are only a fraction of the emissions of a traditional power plant.
Turbine and generator
The steam is led to the turbine at the pressure of 40 bar, and appr. 70 tons of steam is fed to the turbine in an hour. The turbine rotates with the speed of 9000 rpm, and the kinetic energy is transmitted to the generator via the gear box. The rotation speed of the generator is 1500 rpm. The maximum output of the 10,5 kV and 50 Hz generator is 15 MW. A German company, MAN Turbo & Diesel SE has supplied the turbine and generator set of the plant.
The district heating central is situated under the turbine hall, and it consists of two condensers with a nominal output of 40 MW. In the condensers, thermal energy transfers from the hot steam to the cold district heating water. The district heating water is heated from 40-80 degrees to 65-115 degrees depending on the time of the year and the weather. The steam condenses back to water and is returned to the feed water tank and to the boiler to be heated again to steam.
Appr. 4000 kg of bottom slag is formed every hour in the plant. The slag consists of ash as well as metal, stone and glass. Hot bottom slag drops from the grate and is cooled with water under the grate. Next, the slag is automatically extracted onto conveyors which convey the slag to containers. The slag containers are situated in a large container hall where they change automatically when full.
There are two silos (80m3
) in the plant to temporarily store the residues. Boiler ash that has been removed from the heat exchange surfaces of the boiler is stored in one of the silos. Flue gas treatment residue, which is formed when impurities in the flue gases adhere to lime and activated carbon, is stored in the other silo. In addition, there are two larger silos in the plant to store lime and activated carbon.