A LIF and cars investigation of upstream heat loss and flue-gas recirculation as NO x control strategies for laminar, premixed natural-gas/air flames

Abstract

Measurements of temperature and NO mole fraction are reported in laminar premixed natural-gas/air and methane/air flames stabilized above water-cooled sinter and ceramic radiant surface burners. The temperature and NO mole fraction were measured by coherent anti-Stokes Raman scattering (CARS) and laser-induced fluorescence (LIF), respectively. At equivalence ratios of 1 and 1.3, the flame temperature was changed either by varying the degree of heat transfer to the burner surface, through varying the exit velocity of the fuel-air mixture, or by flue-gas recirculation (FGR). At φ=1, the variation in flame temperature with exit velocity observed with the hot radiant burner is indistinguishable from that obtained from the water-cooled sinter, indicating a unique relationship between heat transfer to the burner and actual burning velocity. The variation in NO mole fraction with exit velocity shows marginal differences between the two burners. Plotting the measured NO mole fraction as a function of flame temperature for the surface burners and FGR shows that both control strategies have the same effectiveness in lowering the NO concentration at this equivalence ratio. Consideration of the consequences of these strategies on burning velocity shows that FGR can yield a higher thermal input than upstream heat loss. At =1.3, greater differences are observed between upstream heat loss and FGR as NOx abatement techniques than at φ=1. However, uncertainty in the concentrations of HCN and NH3 in the fuel-rich hot gases, and how they are affected by upstream heat loss and FGR, precludes drawing conclusions concerning the relative efficacy of these techniques. Calculations using GRI-Mech 3.0 show excellent agreement with the results at φ=1 but inadequately predict both the absolute magnitude of the mole fractions and the observed trends with abatement technique at φ=1.3.