We have developed an energy conserving regenerative rotary combustion system (RRX burner) which is combined with a regenerative type air pre-heater and can be operated continuously. In our first report, we discussed "Heat transfer characteristics of the RRX burner" (Kagaku Kogaku Ronbunshu, 1996) and the second report discussed on "Combustion characteristics of the RRX burner" (Kagaku Kogaku Ronbunshu, 1997). We discuss the technology of low NOx emission when the rotary air nozzle exit areas are changed to 88 degrees, 35 degrees and 15 degrees. The results show that the narrower the air exit angle combustion gets, the lower the NOx emission. In the case of angle 15 degrees, NOx emission is less than 9 ppm. We assume that the reaction speed is quite faster than the mixing speed between a fuel, air and flue gas in the furnace, and estimate a flame length and NOx emission based on the velocity and concentration formulas of Beer and Chigier. The estimation results coincide a ell with the experimental results, so it means that flue gas recirculation inside a furnace helps to attain low NOx.