Emission Characteristics of a Premixed Cyclic-Periodical-Mixing Combustor Operated With Hydrogen-Natural Gas Fuel Mixtures

The concept of the cyclic periodical mixing combustion process (CPMCP) [1, 2] for the extension of the lean blowout limit had been implemented in an atmospheric experimental combustor for testing with both external perfect [3] and technical [4] premixing of reactants. It had been tested with natural gas and has now been tested with a mixture of 70%_Vol of hydrogen and 30%_Vol of natural gas (98% CH₄) as fuel. With natural gas the NO_x emissions are unaffected by the limited technical premixing quality, as long as the air preheat is in the design range of the premixers [4]. Then, for adiabatic flame temperatures of up to 1630 K NO_x emissions are below 1 ppm(v) with CO emissions below 8 ppm(v) in the whole operation range of the test combustor (15% O₂, dry). With the “70%_Vol H₂ – 30%_Vol CH₄” mixture the NO_x emissions increase by nearly one order of magnitude. Then, NO_x emissions below 7 ppm(v) (15% O₂, dry) are achieved for adiabatic flame temperatures of up to 1600 K. They approach the 1 ppm(v) level only for flame temperatures below 1450 K. CO emissions are below 4 ppm(v). The reason for the increase of the NO_x emissions is the higher reactivity of the mixture, which leads to earlier ignition in zones of still elevated unmixedness of reactants near the premixer-injector exits. This effect was investigated by chemical reactor network simulations, analyzing a pressure effect and an additional chemical effect of hydrogen combustion on NO_x formation.