Strict regulations of NOx emission standards have resulted in an optimized performance of combustion chambers. The present paper describes the effects of burned gas recirculation on NOx emissions from hythane-oxygen flames in a separated jet burner. NOx emissions from a burner with separated jets and the effects of the size of the burned gases in recirculation in such a burner have not yet been studied. Burners with separated fuel and oxidizer jets permit a high dilution of reactants and a large recirculation zone of burned gases in the combustion chamber, which favors the decrease of nitrogen oxide emissions. In this study, the oxy-fuel burner was equipped with two nozzles: the first nozzle supplied a hythane flow (a mixture of natural gas and hydrogen) and the second nozzle supplied pure oxygen. The hydrogen content in the fuel varied from 0% to 20% in volume. The influence of the distance between the nozzles (12 to 100 mm) and the global equivalence ratio of the mixture were analyzed. Measurements of combustion product concentrations were carried out at the combustion chamber exit using a water-cooled probe and a NOx analyzer. The velocity fields and the size of the recirculation zone were determined by the Particle Image Velocimetry technique (PIV) in the reacting flow. The results showed that an increase in size of the recirculation zone leads to a decrease in NOx emissions (up to 96%), and that NOx emissions are inversely proportional to the measured surface area of the burned gas recirculation zone. Due to the reaction between recirculated CO2 and thermal NO and its chemical effects, NOx emissions decrease in lean combustion because of the destruction of NO. In oxy-fuel combustion occurring in burners with two separated jets, NOx emissions depend on the size and composition of the recirculation zone of the burned gases, which are central to the goal of limiting NOx emissions at the exit of the fumes.
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