In this paper, NO_x emission from the 5-26t/h boilers which are used for the air conditioning or steam source in factory and influence of burning condition on NO_x formation are researched. The main results are summarized as follows.
1) NO_x emission from such boilers is 1-5kg/10⁷kcal.
2) Boiler capacity doecs not influenced on NO_x emission.
3) NO_x emission highly depends on flame temperature. As temperature in higher, NO_x emission more increases.
4) Heat release rate in combustion chamber has same relation.
5) NO_x-Excess air factor relation is divided into three patterns. The pattern that the tested boiler indicated, is determined by the range of excess air factor and fuel-air mixing in combustion chamber.
6) The type of fuel has an effect on NO_x emission. NO_x emission increases in the order of town gas, kerosene and heary fuel oil. On the boilers firing high N content fuel oil, NO_x emission increases. Fraction of N content converted to NO_x is 39%.