The heat flux distribution and emission characteristics of natural gas burners with a range of significantly different mixing characteristics are compared both in a pilot-scale cement kiln simulator and in operating rotary kilns. One burner type tested in the simulator combines a precessing jet and a high-momentum axial jet whereas the other type is the generic multi-channel burner of the international Flame Research Foundation. The precessing jet burner was found to produce higher peak and total heat flux than a multi-channel burner flame optimised to provide good external recirculation, based on the Craya Curtet parameter. However, the NOx emissions from both burners are found to be similar, in contrast to typically large reductions found in industrial installations. This apparent discrepancy is explained by considering the NOx emissions and the heat flux together, since the heat flux is controlled differently in the two facilities. The results demonstrate that the relationship between heat flux and NOx emissions should be considered together in any comparison of different combustion systems.