OCR Text |
Show - 4 - While thermal NO formation predominantly depends upon the reaction temperature and the residence time, fuel NO formation depends upon the oxygen concentration in the pyro-lysis zone of the flame. It is known from earlier investigations [3] that the 'purely* thermal NO -formation in gas-flames can be substantially reduced by optimization of conventional burner parameters, such as swirl, fuel injection, air velocity etc. both with long, lazy - but also with short high intensity flames. At fuel oil and pulverized coal firing success was obtained only with long, lazy flames. This is due to the high proportion of fuel NO in the total emission of oil and pulverized coal firing. Short, intensive flames require rapid mixing between the fuel and combustion air which in conventional burners results in ideal conditions for fuel NO formation (high oxygen concentration in pyro-lysis zone). However, power station boilers with horizontally opposed burners require short, intensive flames. To solve the NO problem in such firing systems it is necessary _rf-to have a burner design which permits a primary zone with high combustion intensity to be operated fuel rich while maintaining the flame length approximately constant by appropriate admission of the residual combustion air, required for burnout. A design principle which satisfies these conditions in an ideal manner is illustrated in Fig. 1. 'Staged mixing' between fuel and air should assist in optimizing the mixing process in the primary and final combustion zones with a view to reducing the formation of fuel NO . 9-4 |