The main purpose of the paper is to find the best condition on reduction of $NO_x$ generated during burning up the municipal waste by using the secondary air injection nozzle. For this purpose, the model incinerator(1/13 scale of real municipal waste incinerator) was made by maintaining the similarity to real municipal waste incinerator and used to make an experiment on reduction of $NO_x$. The model incinerator is composed of the main combustion air injection part, secondary air injection part, combustion chamber, which are the main part of real waste inicnerator, and able to control the temperature in the combustion chamber, the effect of mixing, the effect of injection of reducing agent($NH_3$), which are the important parameters in reducing $NO_x$.
Higher temperature produced higher reduction rate. So in driving the incinerator, getting the highest average temperature in possible temperature range is preferable. In restrict temperature range, the proper secondary air injection nozzle selection as the injection nozzle of reducing agent, $NH_3$ injection nozzle is important and the selection of the secondary air nozzle as injection nozzle of $NH_3$ in fixed S1 : S2 : S3 ratio, too. For the proper selection of secondary air injection nozzle makes the good mixing condition for mixing $NO_x$ with $NH_3$ The proper selection can increase the reduction rate of $NO_x$. In the experiment on change of the $NH_3/NO_{x,initial}$ ratio, the higher ratio, produced the higher reduction rate. Because of low max. temperature, the effect of $NH_3/NO_{x,initial}$ ratio is greater than that of average temperature in the combustion chamber. So the proper selection of $NH_3/NO_{x,initial}$ and secondary air injection nozzle as the injection nozzle of reducing agent $NH_3$ can produce high reduction rate of $NO_x$.
