| OCR Text |
Show 7 3. CALCULATION RESULT 3.1 Effect of Gas Recirculation Increasing the gas recirculation ratio of our model furnace caused the maximum flame temperature to decline and the flue gas temperature at the furnace outlet to rise gradually when there is no heat loss from the recirculating gas, i.e. ex = 1.0. Some heat loss (Le. ex < 1.0) from the recirculating gas has a considerable effect to lower both temperatures, Tfw.z' and Tout. Figure 3 shows the change of these temperatures plotted as a function of the recirculation ratio R in the range of 0.0 to 6.0, and when the heat loss coefficient, ex, is changed in two conditions, 0.5 and 1.0. Thermal efficiency was also calculated as shown in Fig.4 with the same condition as the case of Fig.3. A large amount of recirculation gas does not affect the thermal efficiency so strongly if the heat loss from the recirculating flue gas is relatively small, i.e. less than around 10% loss corresponding to larger ex than 0.9. Heat recirculation by means of preheating the combustion air or fuel results directly in rising the maximum flame temperature. Figure 5 describes the effect of the temperature increase and existing restrictions due to the temperature limit of furnace materials and combustion stability. Available conditions in actual furnace operation may hence be restricted between the 'over temperature zone' and 'blow-off zone'. If we take a specific temperature to distinguish those two zones, for example, 2150 °C for the maximum and 1350 °C for the minimum, heat recirculation brings about an expansion of the available operating range when shown as a function of gas recirculation ratio R. 3.2 Heat and Gas Recirculation Equations (14) and (15) give the maximum flame temperature when the temperature at the heat exchanger exit, 'linl and gas recirculation ratio, R are given. Maximum flame temperature, Tfw.z' was calculated and plotted as a function of 'linl and gas recirculation ratio R with the condition of less ~h than 0.9 indicated as a heat exchanger limit. The contour lines of Tf~ from 1350 °C to 2150 °C, divided into 8 zones, shown in Fig.6. It clearly demonstrates that increasing the gas recirculation ratio without any change of TfmIU is possible if 'lin! can be sufficiently controlled. In addition to this mapping, !1T( = TfmIU - Tout) was also calculated and plotted as a function of 'lin! and gas recirculation ratio, R with the condition of less ~h than 0.9. |
