The current Hamworthy low-NO(x) 3-100 MW dual-fuel register burner design incorporates bluff baffles to induce a circumferentially staged air-flow. It is coupled with a staged atomiser producing a unique flame structure, which demonstrates a high degree of flow staging and an effective means of controlling the formation of NO1. This paper presents the results of comprehensive RFO firing trials, isothermal combustion air mapping, fuel jet trajectory-visualisation trials and flame-temperature profiling, from which the effect of the NO(x)-controlling geometry on the burner emissions and flame structure were deduced. These results are related to the environmental factors created within the flame and believed to influence the mechanisms of NO formation. Finally the implications of the results of the trials and analysis of the results on burner design are discussed.