NO x emission characteristics and aerodynamic structure of a self-recirculation type burner for small boilers

Abstract

To meet the coming, more stringent NO x emission standards for small boilers, a self-recirculation type burner, in which a fresh air is injected through eight nozzles, arrayed peripherally around a liquid fuel-spray nozzle, while the hot burned gas is recirculated within the furnace and entrained into the injected air streams has been experimentally investigated. The flowfield has been determined with a PIV system and spatial distributions of NO x , temperature, O 2 and CO have been determined with use of chemi-luminescence, thermocouple, magnetic pressure type analysis, and non-dispersive infrared analysis, respectively. 2-D flame chemi-luminescence images have been obtained by an ICCD camera synchronized with the PIV system. Results show that the burned gas is recirculated in the furnace and successively entrained into the air streams. As a result, NO x emission is drastically decreased. In the present prototype burner, the amount of a recirculating gas entrained into the air streams attains 50% the fresh air by volume and the NO x level is decreased to 22 ppm from its original value of 108 ppm without recirculation. Further measurements show that besides the recirculating flow outside the air nozzles, another recirculating flow is induced around the center axis inside the injected air streams. Combustion is stabilized around this inner recirculation zone. With an increase in the air flow rate while keeping the spraying fuel flow rate constant, this inner recirculation zone expands and its centre approaches the fuel nozzle. The evaporation of liquid fuel is promoted and a blue flame zone is established. The NO x level is further reduced to 18 ppm when the air excess ratio is increased to 1.6. Thus, the present, simple device of self-recirculation type burner can successfully reduce NO x emission without great cost and space.