Abstract: Lean combustion has been attracting attentions from the viewpoint of 

efficient energy utilization and NOx reduction. We are investigating the possibility of

 ultra-lean combustion (combustion under the condition leaner than the lean

 flammability limit) by using the "swirling stagnation burner" that can form a stable

 recirculation zone of burned gas. In the lecture, the effectiveness and the ultra-lean

 combustion mechanism of this burner will be introduced. In addition, the effectiveness

 of this burner and "the coaxial double tube burner" for the combustion of

 ultra-low-quality fuel gas whose calorific value is too low to be burned by ordinary

 combustion methods, and its combustion mechanism will also be introduced.