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Show 1700 1600 - < r-m*rf\ /': ", • ft . V •X.--*AII> / * Figure 4. Average temperature in freeboard of Kuusamo BFB-furnace The average temperatures in the freeboard in the different modifications are presented in Figure 4. The air distribution in the furnace in each computational case is presented in Table 2. In the existing situation (Case 1), volatile gases are assumed to be released over a relatively small area (3.4 m2) near the fuel feed grounds in observations through the D I M A C furnace cameras. The modelled temperature field can be seen in Appendix 4. According to the D I M A C cameras and modelling results, the combustion is concentrated to the front part of the furnace. The main reasons for this unbalance are clearly the unequal fuel feed and the relatively weak horizontal mixing. Therefore, as the first modification, the effect of more homogenous fuel feed was tested. In this case (Case 2), the droptubes were removed and volatile gases were released from a larger area (5.4 m 2 ) of the bed surface. The result of the modification is a more efficient mixing of fuel and air above the bed and also the combustion is more intensive and homogenous in the lower part of the freeboard. Radiation to the bed increases and thus the bed temperature rises. Table 2. Staging of combustion air Case 1 2 3 4 5 6 7 Total kg/s 12.4 12.4 12.4 12.4 12.4 12.4 11.3 Bed air kg/s 5.0 5.0 5.0 4.3 5.0 5.0 3.8 1 st level kg/s 4.1 4.1 4.1 5.7 0 5.2 4.1 Secondary air 2nd level kg/s 2.2 2.2 2.2 2.4 5.6 2.2 2.2 3rd level kg/s 1.1 1.1 1.1 0 1.9 0 1.1 In Case 3, in addition to the fuel feed modification also the refractory structure is lowered by 0.8 metres. This modification does not affect, in any noteworthy way, to the temperatures in the lowest part of the freeboard or in the bed. Instead, it makes the peak temperature at the 2nd air level 50 K lower. The temperature difference consequent upon the refractory modification disappears after the 3rd air level (about 5 m from the bed surface). In present situation, a relatively large proportion of combustion air is fed into the furnace as fluidisation air. The fluidisation velocity has been kept quite high in order to compensate the unequal fuel feed by intensifying mixing in the bed. Unfortunately, the extra air that is not necessary for combustion in the bed just makes the bed colder. A more equal fuel feed decreases the need for extra fluidisation air. Another possibility to lessen airflow through the bed is to make the bed area smaller. A s a third modification, in addition to the fuel feed modification, the bed area was reduced by filling 0.6 m from the rear wall (Case 4). The fluidisation velocity was kept the same as in reference case, and so the airflow through the bed was reduced. The total airflow to the boiler was kept as a constant, but the distribution of combustion air has been changed. The combustion air staging in each modification is presented in Table 2. This modification raises the temperatures of the bed and the lower part of the freeboard. The temperature at the 1st air level rises because of the more effective combustion that results from the increased 6 |