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

摘要

The concept of the cyclic periodical mixing combustion process (Kalb, and Sattelmayer, 2004, "Lean Blowout Limit and NO_x-Production of a Premixed Sub-ppm-NO_x 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 NO_x-Production of a Premixed Sub-ppm-NO_x 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 (Briickner-Kalb, Hirsch, and Sattelmayer, 2006, "Operation Characteristics of a Premixed Sub-ppm NO_x Burner With Periodical Recirculation of Combustion Products, " Proceedings of the ASME Turbo Expo 2006, Paper No. GT2006-90072) and technical (Brueckner-Kalb, Napravnik, Hirsch, and Sattelmayer, 2007, "Development of a Fuel-Air Premixer for a Sub-ppm NO_x 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% CH_4) 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 (Briickner-Kalb, Napravnik, Hirsch, and Sattelmayer, 2007, "Development of a Fuel-Air Premixer for a Sub-ppm NO_x Burner, " Proceedings of the ASME Turbo Expo 2007, Paper No. GT2007-27779). 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% O_2, dry). With the "70%_(vol)H)_2-30%_(vol)CH_4" mixture the NO_x emissions increase by nearly one order of magnitude. Then, NO_x emissions below 7 ppm(v) (15% O_2, 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 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.