Reaction mechanisms
Analysis and
some important mechanisms
Analysis of reaction mechanisms
• Sensitivity analysis
– Identifies the rate limiting reaction steps
• Reaction pathway analysis
– Determines the characteristic reaction paths
• Eigenvalue analysis
– Determines the characteristic time scales
– Directions of the chemical reactions
• Reduced mechanism
– From info above, can eliminate unimportant reactions
– Generate simplified, or reduced mechanism (for faster
computation)
Explosion limits (H2-O2 system)
• Needs detailed chemistry to understand explosion limits
Following 500C vertical line
• Above 1.5mmHg; explosion
– H3-H6 prevailing over
radical destruction at wall
– Increasing pressure
increases conc & rate
• Above 50mmHg;no explosion
– Competition for H atoms
between H3 (chain
branching) and H11 (chain
terminating)
• Above 3000mmHg; explosion
– New reaction (H16) activated
& adds a chain branching
step
Methane combustion
• Methane mechanism is available as GRI-Mech
(at version 3 now) (325 reactions, 53 species)
• Reaction pathway analysis
– Each arrow shows set of elementary reactions
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•
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•
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Primary reactants at tail
Primary products at head
Additional reactant species along arrow
Coresponding reaction number indicated
Destruction rate of reactants in brackets
– Width of arrow shows significance of the reaction path
NOx formation
• Three chemical mechanisms for NOx
formation
– Thermal (Zeldovich) mechanism
• Dominates at high temperatures (>1800K)
– Prompt (Fenimore) mechanism
• Important in rich combustion
– N2O intermediate mechanism
• Important in very lean, low temperature process.
• Equiv ratio < 0.8