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Show 3. CASES "r \ v~- '''' h^i&&M> 3.1. Vanaja 60 MWpe,, BFB1 The Vanaja power plant is Finland's first thermal power plant built in 1938. The power plant consisted of two grate-fired boilers, a gas turbine with a heat recovery unit and a heat accumulator. The grate-fired boilers were built in the early 1950s, and both of them had a 60 M W ^ fuel input. In 1996 one of them was decided to be converted into a B F B boiler. The old chain grate was removed and replaced with a fluidised bed with appropriate fluidisation air and biofuel feeding systems. At the same time the automation system was modernised. The primary fuel was changed from hard coal into peat with a relatively low moisture content of 40 %, whereas coal remained as the secondary fuel together with r"~ natural gas and biofuels. Figure 2. Structure of the 60 MW^a, BFB furnace The modified boiler is a two-pass construction. The first pass is the furnace equipped with masoned evaporator walls, one curtain evaporator and two superheater blocks connected in parallel without any water spraying steam temperature control system. The adiabatic refractory structure reaches up to the secondary air level, above which two fuel droptubes are placed. The second pass consists of three economiser packages and tube type air preheater. The schematic description of the furnace is presented in Figure 2. The geometry and the main dimensions of the furnace are presented in Figure 3. The Vanaja plant will, besides being in normal commercial operation, be used as a test plant for testing new fuels and developing the fluidised bed combustion. Due to a fundamental change in the combustion technique and a high increase in the fuel moisture content, the air staging and heat release in the freeboard section had to be completely redesigned. The main focus was laid on avoiding any increase in the amount of unbumed fuel and heat transfer to the convective superheater and the economiser tubes. The assignment was a typical design project, where several optional constructions have first to be created and from which the most promising ones are then selected to further investigation. The thermal and combustion engineering design was carried out with aid of A R D E M U S . The effect of the modification on the operating conditions of the turbine plant was calculated with the S O L V O * program that is based on the mass and heat balances. Cold model tests and experimental knowledge completed the design process. Table 1 shows the measured and computed characteristic operation values of the boiler before and after the modification. These represent the full load condition in the measured reference state with coal grate firing, the A R D E M U S simulations with water-steam side values obtained from S O L V O ® and the actual measured values after B F B modification. The results of the simulations were used to study the optimum location and nozzle velocity of O F A . The original construction included additional combustion air located just above the grate and almost all the air was fed through the Figure 3. Dimension of the modified boiler Operated and owned by Imatran Voima Oy 3 |