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Show 6 100 100 Moreover, the volume of the Naantali furnace ( M W / m 3 ) is small, which causes the overall temperatures of the furnace to be high. In conclusion, there are three main reasons for the severe slagging ofNaantali Unit No. 3 boiler, when using the high temperature low-NOx burners and the over fire air system: the burner zone heat release rate is too high for this kind of combustion system causing high temperature areas near the furnace walls, also, the comer structure is not an optimum causing burner jets to bend rapidly to the walls, and there is a high concentration of coal particles near the walls, because of the centrifugal forces. In addition to these, the Naantali boilers have always had some slagging problems, due to the old structure of the evaporator walls (pipes are not connected to each other with tailfins) and the many non-cooled areas (bricks) in the furnace. 3.4 Solution to the slagging problem Picture 3. Burner zone heat release rates In order to prevent slagging in Naantali Unit No. 3, it was decided to decrease the burner zone heat release rate by increasing the space between the burners. The modification of the comer structure was impossible to undertake because of the limited time during the summer overhaul and the high costs. T o decide on a new burner arrangement and to check if the planned modifications would be enough to prevent severe slagging, two different burner arrangements were studied by using A R D E M U S . The existing combustion system in Naantali consisted of three levels of coal burners and two levels of oil burners arranged so that the fuel of the lowest and the highest burner level was oil and the coal burner levels were situated between the oil levels (Appendix 4). The two possible burner arrangements were as follows: the highest coal burner and the highest oil burner changed places as well as the lowest coal burner and the lowest oil burner changed places (Design 95a), whereas in a second alternative arrangement only the highest coal burner and the highest oil burner changed places (Design 95b). On the basis of the simulation results (appendix 2), it was decided to carry out the burner arrangement of "Design 95a". Using this construction, it was possible to reduce the burner zone stoichiometric ratio from 2.0 M W / m 2 to 1.0 M W / m 2 . The simulation results of the new construction showed that the temperatures decrease significantly especially on the lowest burner levels. Both peak temperatures and overall temperatures were reduced, and, most importantly, the temperatures near the walls were reduced to temperatures below 1200 C. |