OCR Text |
Show 9 4.3 Simulation results Effect of the OFA and flue gas recirculation on the furnace temperature fields The overall temperatures and the peak temperatures of the hard coal cases can be observed to be, according to Appendix 5., several hundred degrees higher than in the lignite firing. This mainly derives from the amount of water vapour, which is much higher in the case of o the lignite firing, resulting in maximum flame temperatures below 1200 C. The use of over fire air (OFA) influences the temperature fields of the furnace (Appendices 5 and 6) so that the peak temperatures will be lower than in the case without the O F A. However, the overall temperatures are higher, and the combustion is carried out in the lower part of the furnaces. Combustion using O F A is more vigorous, and the flame temperature is high, because the air factor in the combustion zone is near the one (^=1), which is the theoretical optimum. Because of the higher flame temperature and thus higher furnace absorption, the use of O F A decreases the FEGT. In this case, the F E G T with O F A is 20 C lower than the F E G T without O F A (Table 5). The effect of O F A on F E G T depends on the mixing conditions of the combustion gases and the over fire air and the distance between the highest burner level and the O F A nozzle. According to the operational experience, the difference between the use of O F A and the operation without O F A on F E G T is typically 20-50 °C. Table 5. Furnace exit gas temperature in computed cases Flue gas Fuel input FEGT (°C) flow (kg/s) (MW) Lignite Coal + low-NO*- burners Coal + low-NOx-burners + FGR Coal + low-NO*- burners + OFA 202 139 184 139 315 315 314 317 960 1009 987 990 The use of flue gas recirculation (FGR) increases the amount of gases in the furnace. In the calculations, this is presented in Appendices 5 and 6. The amount of recirculated gases is 3 2 % of the total flue gas flow, and the recirculated gases are taken into the furnace after the electrostatic precipitator (cold gas recirculation). Coal + low-NOx-burners + OFA + FGR 184 320 948 The injection of cold flue gases into the furnace decreases the peak and overall temperatures and thus decreases the furnace absorption. In addition, the temperature fields show that the high amount of F G R causes poor flame stability, and thus the N O x and unburned carbon (UBC) in fly ash will be higher than in the case without F G R . As a result of the calculation, the amount of the F G R should be lower than 3 2 % of the total flue gas flow in order to attain an acceptable flame stability and low emission values. |