Study of low-temperature combustion in a low-NOx burner

A Natural Gas Premixed Forced Internal Recirculation Burner (FIR) developed at the Institute of Gas Technology (IGT) reduces NOx emission level to sub-10 vppm. The FIR burner incorporates air staging and internal recirculation of combustion products, which greatly enhances heat removal. In the primary zone, combustion of rich natural gas/air mixtures occurs at low-temperatures (800 to 1300K), therefore, NOx formation is unaffected by the thermal NOx mechanism - the main source of NO in typical natural gas burners. In the primary zone of the FIR burner, the composition of NOx emissions mainly consists of (70 to 80%) NO2. This composition significantly differs from typical natural gas burners and from the FIR exit composition, where NO2 concentration is less than 5% of the total NOx amount. The low-temperature (800 to 1300K) ignition and combustion in the primary zone of the FIR burner was modeled by admiring products with initial methane-air mixture. The developed approach predicts methane ignition and oxidation within typical residence times in the primary zone of the burner. NOx formation in low-temperature methane combustion was analyzed and attributed to the prompt NO mechanism. High concentration levels of HO2 in the rich low-temperature flame in the FIR primary zone resulted in NO conversion to NO2. To further investigate low-temperature methane combustion a plausible kinetic mechanism was developed. Using the developed mechanism, the ignition of methane was found to be strongly promoted by the internal recirculation of combustion products. When the recirculation ratio is about 20 to 30%. the difference in ignition delays between methane and natural gas is insignificant.