Brief Communication
The Paramagnetism of Soot Particles in
Propane–Oxygen Flames
Z. A. MANSUROV, E. K. ONGARBAEV, AND T. T. TUTKABAEVA
Department of Chemical Physics, al-Farabi Kazakh State National University, Karasai Batyr Str., 95,
Almaty, 480012 Kazakhstan
INTRODUCTION
Investigating the kinetics of soot formation dur-
ing the combustion of a hydrocarbon has signif-
icant scientific and practical interest. One prob-
lem is that soot from the low-temperature
combustion of hydrocarbons has polycyclic aro-
matic hydrocarbons (PAHs) adsorbed on its
surface and consequently has carcinogenic ac-
tivity. In spite of all the work [1– 4] there is still
little clarity on the embryos participating in the
formation and growth of soot particles. Quanti-
tative data on the paramagnetism of soot parti-
cles are lacking, because of the experimental
difficulties of sampling; also the high reactivity
of the active centers on soot particles creates
ambiguity about their identification. It is impor-
tant [5] to study the kinetics of the initial stages
of soot formation; so far it is clear [6] that
“young” soot particles are more reactive than
the “old” ones, with the amplitude of an elec-
tron spin resonance (ESR) signal having been
taken as an indicator of reactivity. This work
investigates the paramagnetism of soot particles
sampled from different points of the flame
front, when burning propane and oxygen.
EXPERIMENTAL
This paper uses ESR spectroscopy to study soot
particles, produced in rich flames of propane
and oxygen, when stabilized in a quartz reactor,
with two separately heated sections. The reactor
and flame stabilization on a matrix have been
described before [4]. The experiments were
done at 1 atm with the first section’s tempera-
ture being T
1
= 423 K; the temperature, T
2
, in
the second section varied from 923 to 1023 K.
The total consumption of fuel and oxidizer was
6800 cm
3
/min with [C
3
H
8
]/[O
2
] = 1. The resi-
dence time in the second section was 1.35 s.
Sampling was done with a quartz probe, in-
serted into the second section of the reactor; the
inlet diameter of the probe was 50 m.
ESR spectra were registered using the spec-
trometer JES-Me-3x with the sample in a spe-
cial cuvette cooled to the boiling point of nitro-
gen. Measurements of the g-factor and splitting
were done in comparison with Mn
+2
ions
against a background MgO. The number of
spins was determined in comparison with a
monocrystalline sample of CuSO
4
5H
2
O of
known mass.
RESULTS AND DISCUSSION
The temperature profiles along the reactor’s
length with a flame of propane and oxygen are
shown in Fig. 1. Soot particles were sampled
from the luminous zone represented by the
shading. The ESR spectra of low-temperature
soot sampled from various points in a flame
front are given in Fig. 2. These ESR spectra are
symmetrical singlet signals, characterized by a
g-factor of 2.003 0.001. Its shape for all
samples is symmetrical about the center and is
described well by a Lorentz function. The width
of the modulation was 0.2 mTl. The value of the
g-factor and the small width of the modulation
indicate that the signal was caused by organic
paramagnetic compounds; moreover, the close-
ness of the g-factor to that for a free electron
(2.0023) shows that the signals are associated
with the products of low-temperature (973 K)
pyrolysis of organic compounds [7]. According Corresponding author. E-mail: mansurov01@yahoo.com)
COMBUSTION AND FLAME 118:741–743 (1999)
© 1999 by The Combustion Institute 0010-2180/99/$–see front matter
Published by Elsevier Science Inc. PII S0010-2180(99)00031-0