Tutorials Manual

Chemkin 4.1.1
Chapter 2: Combustion in Gas-phase Processes
The A-factor computed from the collision frequency between A4 molecules is of the
order of 10 12 and is much larger than the one used in the above nucleation reaction.
2.8.5.2 Soot Mass Growth Reactions
One of the advantages of the Particle Tracking Module is that soot mass growth and
oxidation reactions can be provided as regular surface reactions. For example, the H-
Abstraction-C 2 H 2 -Addition (HACA) soot growth sequence proposed by Frenklach and
coworkers 25,26 can be given as:
(S2) H+H(S)=>(S)+H2 4.20E13 0.0 13000.0
(S3) H2+(S)=>H(S)+H 3.90E12 0.0 9320.0
(S4) H+(S)=>H(S) 2.00E13 0.0 0.0
(S5) H(S)+OH=>H2O+(S) 1.00E10 0.734 1430.0
(S6) H2O+(S)=>OH+H(S) 3.68E08 1.139 17100.0
(S7) C2H2+(S)=>H(S)+2C(B)+H 8.00E07 1.56 3800.0
It is easy to see that each sweep of the HACA growth sequence will increase the soot
particles by two classes.
Marr 23 (see p. 78) suggested that PAH condensation on the soot particle surface can
have significant contribution to soot mass growth in post flame zone. He also found
from his experimental study that the collision efficiency of PAH condensation is of the
order of 0.1. Since a description on how PAH species interact with a soot particle is
not readily available, the PAH condensation reactions used in the simulation are
purely hypothetical. Only condensations of major PAH species in the gas mechanism
are considered. The reaction orders are determined by fitting the experimental data
while keeping the sticking coefficients within the order of 0.1. Basically, the PAH
condensation reactions used here are designed to increase soot particle mass in two
ways: they grow the soot particle by the addition of PAH species and they remove
some H atoms (H(S)) on the soot particle surface so that the more effective soot
growth reaction, C 2 H 2 addition (S7), can proceed at a greater rate.
© 2007 Reaction Design 88 RD0411-C20-000-001