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Show 7 Table 3. Operational data of Naantali unit no 3 Caused by slagging 1993- 1994 1994- 1995 1995- 1996 However, on the highest burner level, high temperatures were still after the modification assumed to occur, and some slagging was expected to build up there. The reduction in the burner zone heat release rate was carried out during the summer overhaul of 1995, and after the modification the slagging reduced significantly, which can be clearly observed in Table 3. As was predicted on the basis of the simulation results, some slagging still built up on the highest burner level and the O F A level. However, the deposits had not completely melted, and they were easily removable. Power reduction (hours) Outage (hours) TOTAL 58 959 1017 12 12 4 Fuel change from lignite to hard coal 4.1 Objective of the study In the second case, ARDEMUS was used to analyse the furnace behaviour of two identical power plants in eastern part of Europe, both of which consist of two identical 135 M W e units. The natural circulation type boilers produce 420 t/h of steam per unit (137 bar and o Table 4. Properties of lignite and polish coal 540 C). The original fuel of the boilers Total moisture (%, ar) L H V (MJ/kg) Ash (dry,%) Volatile content (dry,%) Fixed carbon (dry,%) FR-ratio (-) Lignite 37...41 6.8...7.4 40...42 - - 0.6...1.0 Polish coal 10 26 16 28 56 2.0 Picture 4. Burner cell arrangement is lignite, which, in the future, will be changed into Polish hard coal (Table 4). At the same time, the original slot burners, intended to b u m lignite, will be changed into low-NOx burners. The boiler is supplied with lignite from the six fan mills; one mill is supplying lignite to one cell of the burners. Four of the burner cells are located near the comers, and two of them are located in the middle of the furnace walls (Picture 4.). The burner jets are introduced to the furnace by directing them tangentially to an imaginary circle in the centre of the furnace (tangentially fired boiler). In the full load of the boiler, all six burner cells are in operation, and, in the low load operation, less than six burner cells are in operation. At the time when the fuel will be changed into hard coal and the burners |