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 (Kalb, and Sattelmayer, 2004, “Lean Blowout Limit and NOx-Production of a Premixed Sub-ppm-NOx Burner With Periodic Flue Gas Recirculation,” Proceedings of the ASME Turbo Expo 2004, Paper No. GT2004-53410; Kalb, and Sattelmayer, 2006, “Lean Blowout Limit and NOx-Production of a Premixed Sub-ppm-NOx Burner With Periodic Recirculation of Combustion Products,” ASME J. Eng. Gas Turbines Power, 128(2), pp. 247–254) for the extension of the lean blowout limit had been implemented in an atmospheric experimental combustor for testing with both external perfect (Brückner-Kalb, Hirsch, and Sattelmayer, 2006, “Operation Characteristics of a Premixed Sub-ppm NOx Burner With Periodical Recirculation of Combustion Products,” Proceedings of the ASME Turbo Expo 2006, Paper No. GT2006-90072) and technical (Brückner-Kalb, Napravnik, Hirsch, and Sattelmayer, 2007, “Development of a Fuel-Air Premixer for a Sub-ppm NOx Burner,” Proceedings of the ASME Turbo Expo 2007, Paper No. GT2007-27779) 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 $NOx$ emissions are unaffected by the limited technical premixing quality, as long as the air preheat is in the design range of the premixers (Brückner-Kalb, Napravnik, Hirsch, and Sattelmayer, 2007, “Development of a Fuel-Air Premixer for a Sub-ppm NOx Burner,” Proceedings of the ASME Turbo Expo 2007, Paper No. GT2007-27779). Then, for adiabatic flame temperatures of up to 1630 K $NOx$ 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 “$70%volH2−30%volCH4$” mixture the $NOx$ emissions increase by nearly one order of magnitude. Then, $NOx$ 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 in the $NOx$ 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 $NOx$ formation.