Abstract: Regarding the problem of high NO_x concentrations in a 330 MW boiler with quadrangular circular firing and hot air coal-powder feeding, based on the analysis of the impacts of tertiary air on NO_x generation, the key components of the combustion system are optimized. To be more specific, the low level nitrogen burner with horizontal primary air separation, plus the tertiary air with external separation together with the in-depth air classification technologies are implemented in the entire retrofit scheme. Especially for the tertiary air that may highly affect the NO_x generation, the external separation and vertical space classification and delivery technologies are used. It is evident from the actual operation that after the retrofit, the NO_x generation volume is reduced significantly, while the boiler combustion and operation still remain stable state without any side effect on the boiler efficiency.