The flame appearance, temperature profile, and NOx emission associated with gaseous and liquid fuel furnace combustion are investigated through a series of experiments. Numerical simulations are also conducted, for several cases, in order to complement the experimental data. Three fuels, namely, hydrogen, propane, and Jet-A (C 12 H 23), which is a liquid fuel, are examined. Combustion is carried out in two 840-mm long furnaces with diameters of 95 and 182 mm, respectively. Combustion air is introduced to the furnace through two coaxial air nozzles, which generate air velocity difference ΔUa. The flame appearance, temperature profile, and emission index of NOx (EINOx) of gaseous fuel combustion are found to depend strongly on the furnace diameter, D. The results indicate that gaseous fuel combustion can be characterized according to the furnace volume. In addition, the temperature profiles reveal that larger values of D and ΔUa result in lower temperatures. The EINOx is demonstrated to be well scaled with a parameter DU f ΔUa. Finally, the normalized EINOx is shown to be inversely proportional to DU f ΔUa because of the effects of dilution and flame stretch. However, the vaporization process in liquid fuel combustion appears to weaken the sensitivity of the normalized EINOx of Jet-A combustion on the parameter DU f ΔUa.