Chemical Kinetics: NOx Mechanisms

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1 Mole Fraction Temperature (K) Chemical Kinetics: Nx Mechanisms Jerry Seitzman CH4 H HC x 1 Temperature Methane Flame.1..3 Distance (cm) KineticsNx -1 Nx Formation Already pointed out that Nx is pollutant N leads to acid rain/photochemical smog N converted to N in atmosphere bservation: original Zeldovich mechanism does not accurately predict N production in wet air low T combustion with long residence times stoichiometric and rich H -air flames low residence time or rich hydrocarbon flames Need more advanced N mechanisms KineticsNx - 1

2 Thermal N Mechanism Already described basic Zeldovich mechanism add reaction for wet systems (H species) H R N N N { N.1} 75.1kcal mol N N { N.} 3.1kcal mol H N N H {N.3} 48.7kcal mol Thermal N Formation Extended Zeldovich Mechanism still rate limited by { N.1f } dominates in high T (>18K), lean environments long res usually required (postflame gases) superequilibrium [], [H] increases N prod. rate KineticsNx -3 KineticsNx -4 Intermediate N Mechanism N converted to N then N N M N M { N.4} N H N NH { N.5} N N N { N.6} H R 4kcal mol 35kcal mol 36kcal mol N H N H compete with N N N Formation N H N H {N.4} has Lindemann-like p dependence (k k ) medium to high activation energies E a (kcal/mol)~18{n.4}; 35{N.5}; 3{N.6} important in low T, fuel lean ( <.8) systems (+ high p) NH from {N.5} can also lead to N NH N H NH N H

3 KineticsNx -5 NNH Mechanism Thermal + nitrous mechanisms ok for H -air, BUT N also produced via NNH intermediate H R N H M NNH M { N.7} 6.5kcal mol NNH N NH {N.8} 11.3kcal mol {N.8} has same products as{n.5} NH can also lead to N Where is NNH mechanism (path) important? for T> K, important for stoichiometric and rich, low residence time systems H -air flames for T<19 K, most impact (relative to thermal N x ) near stoichiometric (and at lower T) KineticsNx -6 Fenimore/Prompt Mechanism In thermal, N, and NNH mechanisms, N produced via conversion from attack on N / via radicals containing only N,, H nuclei Sufficient for wet air and hydrogen combustion N can also be produced in hydrocarbon combustion via C species prompt Nx Fenimore observed N formed earlier in HC combustion than possible from thermal mechanism General reaction scheme CH x (e.g., CH, CH and CH 3 ) radicals react with molecular nitrogen to form HCN (hydrogen cyanide) conversion to N through various intermediates 3

4 KineticsNx -7 Fenimore Scheme Early version of the mechanism Rate limiting step CH N HCN N Conversion to N HCN NC H NC H NH C NH H N H N H N H conversion of N conversion to N via radical/radical reactions Limited accuracy (e.g., <1.) General Prompt Mechanism Many reactions for range of rich mixtures/fuels graphical representation (ref. Bowman, 4 th Combustion Symposium) C CH x Fuel-N NH 3 KineticsNx -8 N HCN H,H H CN, H NC H H HNC NH H N NH N N H, N CH x (x=1,,3) also in thermal mech. 4

5 N Production Rate (mole/cm 3 /s) N Mechanism Noted that N converted to N in atmosphere combustion systems also convert N to N can be significant fraction of total combustor Nx emissions [N]+[N ] N H N H { N.1} N H N H { N.} N N { N.3} No N from hot regions N from N mixing into low T regions H low T forward reactions (N destruction) active at high T KineticsNx -9 Example: Premixed Laminar Flame N, prompt important in reaction zone; thermal in downstream Cold Post-flame region.4 reactants Total.3..1 =.85 N +H NH+H N+H Prompt mech. N+ Primary source of N? N+H N + Thermal Mech. -.1 KineticsNx -1 CH 4 /air 1 atm 3K/8 F inlet ms H +N N +H 3 ms 4 ms Time 5