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Show 8 The contour lines of ~T from 100°C to 1000 °C divided into 9 zones, are shown in Fig.7. This mapping implies there is an area where the temperature difference is remarkably small compared to conventional furnace. Both Tf~ and ~T distributions were then superimposed in Fig .8. Here the contour lines of ~T for every 50°C in the range from 100°C to 300 °C are shown. 3.3 Thermal Efficiency The operation to increase heat and gas recirculation while maintaining the maximum flame temperature can be performed as shown in Fig.9, where Tf ..-, Tout and 1inl are indicated. 1inl has to be increased to compensate the Tf~ decay resulting from the effect of dilution due to the combustion gas recirculation. The temperature difference, ~T, gets obviously smaller and Tout is gradually close to Tf..- . The thermal efficiency in this conditions were calculated and the result is shown in Fig.1 o. It revealed this operation can increase the thermal efficiency as well as improving the temperature difference, ~T . 4. DISCUSSION The method of excess enthalpy combustion has been generally interpreted to be one of the effective ways to enhance the thermal field in furnaces for increasing the heat transfer rate. The excessive increase of the maximum flame temperature, however, was a major problem to cause over temperature to materials of the furnaces. The result obtained here as shown in Fig.1 0 revealed high temperature excess-enthalpy combustion can be significantly effective under the condition lowering the adiabatic flame temperature forcefully by the gas recirculation. This improved heating method of combining high temperature excess-enthalpy combustion with gas recirculation makes it possible to achieve a uniform temperature profile in furnaces while holding the maximum flame temperature constant. A profile of thermal efficiency affected by gas recirculation ratio R and maximum flame temperature TfmtU were calculated to determine the optimized operating condition between thermal efficiency and NOx emission. NOx emission is in principle supposed to have a strong relationship with the peak flame temperature. A small amount of cold gas recirculation is actually one of the ways to achieve low NOx level even though it frequently causes unstable combustion, which results from a flame temperature \ |
