OCR Text |
Show 9 decrease. Hence it is assumed for this discussion that NOx level is determined mainly by the maximum flame temperature. The contour profile of the thermal efficiency obtained is shown in Fig.11 (a) based on the assumption that the maximum heat exchange coefficient of inlet air and fuel, ~h' which was defined in equation (2), can be less than 0.9. The lines of constant thermal efficiency in Fig.11 (a) demonstrates that there is a trade-off point in terms of thermal efficiency maximization and NOx emission minimization on the line of the heat exchanger limit line corresponding to ~h =0.9. The lowest TflftlU is found on that heat exchanger limit line for a given thermal efficiency. For example, when a design thermal efficiency was fixed at 0.7, the lowest maximum flame temperature, 1730 °C would be found on the boundary line of the heat exchanger limit with gas recirculation R = 4.5. If flexibility in operating temperatures is more important than minimizing NOx emissions, then it is possible to maintain thermal efficiency by varying the gas recirculation ratio while changing the operating temperatures. Using the previous thermal efficiency as an example, 0.7 can be attained with the maximum flame temperature in ' the range from 1730 °C to 2150 °C by varying the gas recirculation ratio. If the maximum flame temperature was fixed at 1800 °C, it could be attained with any thermal efficiency in the range from 0.45 to 0.75 by controlling the gas recirculation ratio. Thus maximum thermal efficiency can be obtained while minimizing the NOx emission level or constant flame temperature can be maintained. The same profile was plotted with a three dimensional plot in Fig .11 (b) as a reference. High temperature excess-enthalpy combustion combined with high temperature combustion gas recirculation taking place in the above-mentioned condition, should be very unique compared to normal combustion because the temperature increase has to be small due to the heat capacity increase. This unique combustion, defined by the authors as 'High Temperature Spatial Combustion' , may have completely different characteristics from a conventional operating method concerning flammability limits, combustion stability, combustion noise and ignition process beyond the autoignition temperature. This improved heating method using High Temperature Spatial Combustion can provide several advantages such as control of the maximum flame temperature in responding to the properties of heated materials within the available zone and in principle very low NOx levels. |