Experimental study on the influence of the nitrogen concentration in the oxidizer on NO_x and CO emissions during the oxy-fuel combustion of natural gas

In this research, the influence of the nitrogen concentration in the oxidizer, oxygen/fuel equivalence ratio and temperature of the combustion chamber on the combustion characteristics of a natural gas fired oxy-fuel jet burner was observed. The thermal input of the fuel was 600 kW and the nitrogen concentration in the oxidizer was varied between 0 and 15 vol.-%, wherein either the maximum combustion chamber temperature or the thermal output were held constant. It was observed that in case of a constant combustion temperature, even small volume fractions of nitrogen in the oxidizer resulted in non-negligible reductions in the thermal output. Furthermore, the experimental results showed that an increase in the nitrogen concentration from 0 to 15 vol.-% caused an approximately eight-fold increase in the NO_x emissions and a considerable decrease in the CO emissions. The impact of the oxygen/fuel equivalence ratio on the NO_x emissions was negligibly small up to a nitrogen concentration of 2 vol.-%. A reduction of the oxygen/fuel equivalence ratio to near stoichiometric levels resulted in NO_x emission factors below 52 mg/kWh. The influence of the oxygen/fuel equivalence ratio on the NO_x emissions was significantly more pronounced with an increasing amount of nitrogen present in the oxidizer.