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Show 3 3 Solving the slagging problem 3.1 Objective of the study Naantali Power Plant consists of three identical units, whose total fuel inputput is 3 * 315 M W f . The Sulzer-type boilers produce 420 t/h of steam each (HP 180 bar, 530 ° C; L P 42 bar, 540 ° C ) and it is supplied with coal from three coal mills, and the burners are located in the comers (tangentially fired). All the coal burned is mainly imported from Poland and Russia. During the operating period 1994-1995, Naantali Unit 3 suffered severe slagging. The reason for the abnormal slagg formation was the new low-NOx-burners, which had been installed at Naantali Unit No. 3 during the summer overhaul in 1994. In addition, during the operating period 1994-1995, a different kind Polish coal was burned in Naantali and in most of the Finnish power plants. This low-quality Polish coal was reactive, but its calorific value was low and the ash content high (Table 1.). The low-quality coal was also burned in Naantali Units Nos. 1 and 2, where the slagging had increased. Anyway, the slagging did not disturb the power production. Instead, in Naantali Unit No. 3, slagging caused a total of around 1000 hours'reduction in power production during the operating period 1994-1995. Tablet. Fuel analysis Total moisture (%, ar) L H V (MJ/kg) Ash (dry,%) Volatile content (dry,%) Fixed carbon (dry,%) FR-ratio (-) Low-quality Polish coal 12...16 18...20 20...22 31...33 45...48 1.4...1.5 Normal Polish coal 9...13 24...26 12...14 32...35 52...55 1.5...1.6 3.2 ARDEMUS modelling The ARDEMUS boiler model simulations were carried out for Naantali Unit No. 3 to find out the reasons to the severe slagging. The modelling was carried out with the maximum continuous fuel power equal to 315 M W f and corresponding to the 1 0 0 % load condition. In the burner zone the reducing conditions were prevailing and the additional air was introduced to the boiler through the over fire air nozzles. The overall air factor was 1.3 and the low-quality Polish coal was used as a fuel. The computational domain of Naantali Unit No. 3 included the first superheater section after which the outflow boundary condition was placed. The domain consisted of the 90 000 number of control volumes with a refinement in the near burner region. The combination of physical and chemical models for combustion was implemented to the standard solution techniques of Phoenics. Altogether three cases were computed. The first case was the existing situation and the other two were the proposed modifications to the burner arrangement in order to rninimise the slagging. During these calculations, no slagging model was used in A R D E M U S , but the slagging tendency was estimated by using calculated flow fields, particle trajectories and |
