Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 91

injection of propane into hot air, polycyclic aromatic hydrocarbons (PAH) species
are formed before ignition of the mixture, 30 and there is a possibility that these
species play a role in lowering the NO levels by reacting with the nitrogen oxides.
Here the propane ignition in the hot air jet is simulated through a well-stirred reactor
model, and the mechanism of PAH formation is analyzed. Methane, as the simplest
hydrocarbon fuel, could also be considered, but propane flames are expected to form
PAH species more easily than methane flames. This is because in the former flames
the concentration of C 3 species, important PAH precursors, is higher.
Richter et al. 31 suggested a chemical reaction scheme for predicting PAH and
fullerene formation in rich flames of aromatic hydrocarbon fuels. The thermochem-
ical and transport parameters for each species are also provided in this valuable
work. They have tested the scheme with experimental data from a benzene/oxy-
gen/argon flat flame and have concluded that their scheme reproduced the experi-
mental results reasonably well. Here the same scheme (slightly modified by inclusion
of propane oxidation reactions) is closely applied to a well-stirred reactor situation
in which rich mixtures of propane and air are preheated to a high initial temperature
and allowed to react and ignite. The well-stirred reactor is an idealized simulation
target, but it is very useful because its modeling does not require parameters such
as diffusion coefficients, thermal conductivities, and viscosities of each species.
Accurate dependencies on temperature are very difficult to obtain, especially for
species as large as PAH molecules and radicals. Thus, the chemical reaction scheme
and the thermochemical properties of the species included are the only data required
to simulate the system. Although the one-dimensional flat flame model is more
realistic, it has a greater propensity to lead to errors caused by inaccuracies in the
transport parameters. The well-stirred flame model allows study of the flame chem-
istry, isolated from the physical phenomena involved in combustion processes.

2.3.5.1 Calculation Method

The calculations were performed with the SANDIA CHEMKIN 19 package. The
chemical reaction scheme was that suggested by Richter et al. 31 with the addition
of reactions of propane combustion taken from GRI-Mech. 32 The resultant combined
scheme comprises 907 reactions occurring among 251 species.

2.3.5.2 Results and Discussion

Ignition of φ = 5 Mixture
First, a mixture of equivalence ratio φ equal to 5 was investigated. This is a very
rich condition, rarely seen in practical devices. However, because we are interested
in simulating the combustion reactions in a flow of propane injected into hot air,
which forms a diffusion flame in which the equivalence ratio varies from zero to
infinity depending on the position in the chamber and time, the φ = 5 condition
chosen here is reasonable.
In addition, PAH species are known to be formed in the postflame region of a
rich flame, where the local equivalence ratio would be even greater, because the
concentration of oxygen there is expected to be very low.