The effect of fuel-nitrogen functionality on the formation of NO, HCN and NH_3 in practical liquid-fuel flames

摘要

An understanding of the complex reaction mechanisms involved in the formation of nitrogen oxides from the combustion of fossil fuels provides a basis for the design and application of NO_X control strategies. This experimental study is concerned with the formation of HCN and NH_3 (as the dominant NO_X precursors) from 130 kW turbulent-spray flames operating in standard and externally air-staged modes. Detailed nitrogenous species-concentration measurements from a series of nitrogen-doped gas-oil flames (using pyridine, pyrrole, quinoline, benzonitrile, benzylamine and phenylbenzylamine) supported an NO-formation route where fuel-nitrogen is initially converted to HCN, which subsequently decays to NO via NH_3. Although variations were found in the developments in concentration and peak concentration levels of HCN and NH_3 with each additive for an equivalent fuel-nitrogen concentration, the differences in the final NO emissions were small. Comparison of the experimental NO concentration profiles from combustion of gas oil with the same fuel containing 0.45% by mass nitrogen (by doping with pyridine) enabled the developments of thermal and fuel-NO to be followed separately for both combustion modes. For an air-staged flame operating at a primary zone (fuel/air) equivalence ratio φ_1 of 1.21, thermal-NO was reduced by 21 % relative to an unstaged flame at the same overall stoichiometry of φ_2 = φ_1 = 0.85. A fuel-NO reduction of 33% was accompanied by significantly increased in-flame production of HCN.