OCR Text |
Show 8 will be changed into low-NOx-burners, also the six fan type mills will be replaced with four ball mills, each one of which is supplying fuel to one burner level. Also two burner cells (those which locate in the middle of the walls) will be left out of use. When changing the fuel from lignite into hard coal, the furnace absorption will increase, because of the higher temperature of the coal flames resulting in a lower furnace exit gas temperature (FEGT). In addition the flue gas amount will decrease significantly decreasing the rate of convective heat transfer. These phenomena mean that the operation of the entire boiler will change, and modifications must be done in order to maintain the original steam temperatures and to obtain a better boiler efficiency. In this case, an A R D E M U S model was made of this boiler to predict the furnace exit gas temperature before and after the modifications. This new value of F E G T was then used to calculate the boiler water/steam side performance and, as a basis of this, to design needed modifications in the furnace and in the boiler second pass heating surfaces. 4.2 ARDEMUS modelling The modelling was carried out with the maximum continuous fuel power equal to 315 MWf and corresponding to the 1 0 0 % load condition. In the reference, the case combustion occurred in oxidising conditions, and the overall air factor was 1.3. The fuel in the calculation was lignite (Table 4). In the cases after the modification, instead, the reducing conditions were prevailing, and the additional air was introduced to the boiler through the over fire air nozzles with a stoichiometric ratio of 1.3. The fuel was Polish coal according to Table 4. The computational domain of the boiler included the first superheater section, after which the outflow boundary was placed. The grid consisted of the 130000 control volumes with the refinement in the near burner region. The solution techniques of Phoenics were applied. The project begun with ARDEMUS simulations of the existing situation - fuel as lignite and old burners. In the second computed case, the fuel was changed to hard coal and the burners to low-NOx-burners. In this case, the burners were located in the same places as the original lignite burners. W h e n comparing the F E G T values of these two simulations, it was evident that the fuel switching has a big effect on the boiler operation, and some modification must be done in the boiler in order to maintain the existing operation of the boiler. The following proposals for boiler modifications and their combinations were considered: 1) Added flue gas recirculation 2) Lifting of the burners upwards 3) Adding to the superheater area The effect of items 1 and 2 on FEGT and on combustion generally were examined by using the A R D E M U S furnace model. In addition of these, the effect of the over fire air to the F E G T was studied. Item 3 was examined by boiler water/steam modelling package. |