Abstract: A combustor comprises a passage area changing mechanism which relatively moves a flame holder relative to a premixed combustion nozzle in an axial direction so that the area of passage for premixed gas between an injection edge of the premixed combustion nozzle and a peripheral edge of the flame holder can be changed. Stable combustion can be maintained over a wide load range and reduction in NOx can be achieved.

Abstract: First and second premixed burners having annular injection slots are concentrically adjacent and the injection slots are substantially aligned with each other in a longitudinal direction of the burners. A pilot burner is provided substantially in the center of the annular first premixed burner so that the injection slot of the pilot burner is substantially aligned with the injection slot of the first premixed burner in a longitudinal direction of the burner. A flame holder which forms a circulating flow of combusted gas Gh downstream of the flame holder is disposed at the downstream end of a boundary wall between the premixed burners so that the flame holder is co-used by the premixed burners. Flow rate adjusting means for adjusting the flow rate of fuel supplied to the first, second premixed burner and the pilot burner, respectively are provided. The fuel can be substantially uniformly mixed with a combustion gas. Reduction in NOx and stabilization of premixed flame can be achieved.