High air-preheat temperatures have been used widely to increase thermal efficiencies in industrial processes. The higher flame temperatures generally result in greater NOx emissions. In this study the distribution of temperature and the concentrations of major chemical species were determined for nominal 586 kW thermal input turbulent-diffusion flame burners firing natural gas and air, and the burners operated in an unstaged or air-staged mode. Experimental results indicated that raising air-preheat temperatures from 800 to 1250 K increased NOx emissions by a factor of 11 for the unstaged burner and 5 for the staged burner. By means of air-staging, a reduction in NOx emissions levels of approximately 50-60% was achieved for air temperatures within the range 1160-1300 K. These reductions were less at lower preheat temperatures. Changing the firing rate over a moderate range of 25% did not significantly affect NOx emitted. Numerical computations of temperature and species concentration (including NOx) in a regenerative burner and combustion chamber are compared with experimental results. A good qualitative agreement was obtained between experimental results and theoretical prediction. With the numerical model used in this study, it is possible to make reasonable predictions of NOx emissions for burners of different geometries. The difference between the measured and predicted NOx emissions is 22%. However, the simulation provides good trend studies to investigate the effects of burner geometry and input variations. This indicates that the combination of experiments and modelling can be considered a generally useful method of solving complex engineering problems.