| Popis: |
Equilibrium considerations indicate that the concentration of total fixed nitrogen species (other than molecular N 2 ) should reach a minimum under fuel rich combustion conditions. Increasing the combustion temperature should alleviate the kinetic constraints that prevent the fixed nitrogen concentration from approaching the low values predicted by equilibrium. In this study the concept of fuel rich combustion at high temperatures was explored, and the dependence of fixed nitrogen conversion to N 2 on the equivalence ratio, temperature and reaction time was determined in oxygen-enriched methane/air combustion doped with ammonia or nitric oxide. To maintain constant, high temperature conditions required for the fixed nitrogen to N 2 conversion, a tubular flow reactor with its walls heated to the adiabatic temperature of the premixed flames supplied by a flat flame burner was used for these reaction studies. The results indicate the validity of the concept of minimizing fixed nitrogen species (HCN+NH 3 +NO) concentration by increasing the temperature in the range of 1850 K to 2050 K at the optimum equivalence ratio of 1.7–1.9. The predominant initial nitrogen species is HCN which is converted to N 2 presumably through amine and NO intermediates. A comprehensive treatment of the mechanism of nitrogen chemistry in rich combustion led to a kinetic analysis of the results, which indicates that near the flame front fixed nitrogen conversion to N 2 follows a second order rate law because of the rate controlling nature of bimolecular reactions involving fixed nitrogen intermediates (NH 4 , N, NO). At the same temperatures, but longer reaction times HCN reaction with chain carrier radicals becomes rate controlling, as evidenced by the first order decay of HCN. |
