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Show content produced superheated steam temperatures of over 400 °C and an unacceptably high steam content in the economiser. Fortunately, the high air factor was used in grate firing. So, by decreasing the air factor to the moderate value of 1.26 with 4 0 % moist peat, the flue gas flow was estimated to be below the original which lead to the calculated temperature of superheated steam equal to 390 °C. As can be seen in Table 1 this agrees well with the measured value of 386 °C. This is slightly lower than the calculated one, which is most probably due to greater flue gas flow, because the leakage of the furnace was not considered. Although, it is well below the designed operating point of 400 °C, it gives an beneficial safety margin, because the peat moisture content usually exceeds 4 0 % , and the superheating circuit is not equipped with a water spraying system. The modified boiler was commissioned successfully at the beginning of 1998, which is the most evident proof of a proper design. The CFD-based combustion modelling was applied to studying and designing qualitatively various optional constructions and to assuring a qualitatively correct operation of the furnace. The computed values were within the satisfactory limits, and, in spite of some quantitative inaccuracies involved, notable benefits were achieved. Moreover, in reality the actual instantaneous absolute operating conditions like fuel properties and air and fuel mass flows are not accurately known and may change considerably. 3.2. Kuusamo 30 MWpeat BFB1 In Kuusamo, a small-scale combined heat and power plant was started in 1993. Its electric output is 6 M W and district heat output 20 M W . The district heat is supplied to the municipality of Kuusamo. In addition to power production I V O demonstrates its new bed mixing dryer technology for moist fuels at this plant. The basic power plant concept is very simplified in aim to keep the investment low, which means that no air or feed water preheaters except the feed-water tank are used. Peat and wood waste are used as a fuel of the bubbling fluidised bed boiler. The moisture content of fuel is approximately 50 % in the state of delivery and 15 % after the dryer. The combustion air injected to freeboard is distributed to three levels, and the refractory structure covers walls up to the second air level. The wet fuel is injected to the front part of the furnace through two droptubes just above the bed surface. W h e n the fuel dryer is applied, dried fuel and bed material used in drying are injected into the furnace through two openings located in the front wall between the droptubes. The control of the bed temperature has turned out to be a problem at Kuusamo. According to measurements, the bed temperatures were usually very low, but still in the bed there occurred some sintering. A R D E M U S was used as a troubleshooting tool in search for the causes and remedies for this problem. As a reference case the existing situation was analysed. Then, a variety of possible furnace modifications were computed. The focus was laid on the effect of the modification to the temperatures in the bed and the lowest part of the freeboard. The aim was to raise the average bed temperature but to avoid high temperature peaks in the freeboard near the bed surface that could cause softening and sintering of sand. In this study, only the radiation from the freeboard and the amount of cold primary air were considered to be changing parameters in the energy balance of the bed. The amount of fuel burning in the bed was kept as constant in every computation case. Every modification case was first computed by using 700 C° as the hypothetical bed temperature. The radiation taken from this computation was used in the energy balance, from which the bed temperature is computed. The furnace iteration could be continued by using this new bed temperature, but because of long computation times, in this study, this result was accepted to be good enough to show the direction and relative magnitude of the change in temperature. The inaccuracies and the limitations of the models have to be remembered, when the final results are discussed. Because of this, in this study, the main attention was paid to the direction of changes instead of the exact operating conditions. Because the problems concerning the bed temperature originated from the furnace design, and the fuel dryer had no actual effect on the bed temperature, only the situation without fuel drying was studied. In all computed cases, peat with a 50 % moisture content was used as a fuel, and the fuel input was 27 M W . 1 Owned by IVO and turnkey delivered by IVO Power Engineering Ltd. 5 |