NO_x reduction effect of two-stage combustion using tubular flame

Abstract

The recuperative burner and the regenerative burner that applied exhaust heat recirculation are proposed as energy-saving technology in furnaces in a practical use. Although, heating efficiency has the proportional to the rate of the exhaust heat recirculation, the burned gas temperature and NO_x emission increases in the conditions of high exhaust heat recirculation. The newly developed two-stage combustion method was proposed in this research for improving NO_x reduction. The two-stage combustion method is often applied as low NO_x technology with using exhaust heat recirculation. This research proposed a tubular flame for the first stage of two-stage combustion. Since the tubular flame can sustain a stable premixed flame in rich mixture in the first stage, NO_x generated in the primary burner suppresses due to low flame temperature in this proposed system. In addition, the inlet gas temperature in the secondary burner can be adjusted by intermediate cooling section that placed between the primary burner and secondary burner. This reduction of the inlet gas temperature is also contributed to reduction of NO_x in the secondary burner. Results show that the first flame was successfully formed until equivalence ratio 1.7 by using tubular flame in the primary burner. The smallest NO_x emission appeared in the condition that the adiabatic flame temperature become equilibrium to the amount of heat reduction calculated from the intermediate cooling section. The NO_x emission is reduced to 16.3 ppm (O₂=0%) at the equivalence ratio of 1.67 in the primary burner without intermediate cooling tube. This value is about 1/6 of the single-stage combustion with Bunsen flame, and about 1/4 of the two-stage combustion with partially premixed flame in the primary burner.