The Effect of Diluents on the Formation Rate of Nitrogen Oxide in a Premixed Laminar Flame

Abstract

New high-efficiency power cycles and environmentally friendly cycles have introduced combustion atmospheres that differ from the traditional hydrocarbon-air mixtures. Wet cycles, solid oxide fuel cell with a gas turbine (SOFC-GT), CO2 separation/capture and biogas combustion are processes that involve high concentrations of inert gases such as H2O, CO2 and N2. These new combustion atmospheres have not been well characterized for premixed flames, hence greater interest is attached how NOx formation is affected. At combustion temperatures above 1800 K, NOx emission is dominated by thermal NOx. The thermal NOx mechanism consists of three elementary reactions. The process is known to be exponential in combustion temperature, but it is also comparably slow and thus dependent on the residence time and the temperature in the post-flame zone. To model the flame, code for a one-dimensional flame with detailed chemistry was used. The flame code solves the combustion evolvement for a one-dimensional, premixed laminar flame. Detailed chemistry was used to model the chemical kinetics. NOx production was described by a NOx mechanism, including thermal, prompt and N2O intermediate. Altogether, the mechanisms consisted of 116 species and 713 reactions. The cases investigated were all premixed flames, diluted with either H2O, CO2, N2 or Ar. The cases used a constant combustion temperature of 2000 K and different pressure levels. All cases were investigated at constant inlet air-fuel temperature and varying equivalence ratio. The rate of formation of NO was investigated for both natural gas and hydrogen flames. The rate of formation of NO is reduced by the addition of any diluents at constant combustion temperature if the O-atom concentration is reduced in the high temperature post-flame zone. The computations show equilibrium between O and O2, and the reduced rates of formation of NO (at constant temperature) are thus simply the result of reduction in the product [O2]0.5[N2] in the post-flame zone.