Environmental Engineering Reference
In-Depth Information
This shows that initially, the fuel is consumed in the inner, rich, premixed
ame,
forming intermediates and CO, which undergo further complete combustion to form
CO
2
. The difference in the concentrations is due to the fact that propane has C
fl
C
-
bonds in addition to C
C bonds being weaker, initiate the chain
reactions sooner for propane, and thus, the auto-ignition temperature of propane is
lower than that of methane.
Gore and Zhan (
1996
) measured the
H bonds. The C
-
-
flame heights, mole fractions of pollutants
and other major stable gas species like CO
2
, CO, H
2
,O
2
,N
2
and C
2
H
2,
in a fuel-
rich mixture of methane and air, in an over-ventilated co-
fl
fl
ow of secondary air. The
overall equivalence ratio was
ame
length and luminosity decrease with increasing amounts of premixing air. The
EINO
x
emission index has a value of 1.1 g/kg of fuel for a pure diffusion
fixed at 0.5. They observed that the visible
fl
fl
ame,
which remains almost constant near this value till the premixing air reaches
U
p
¼
4.
It is because the decreasing
flame height accounts for the decrease in the residence
time and thus a decrease in the rate of production of thermal NO
x
. An increase in
temperature as indicated by the peak in CO
2
and H
2
O mole fractions (more com-
plete combustion) and a broadening of the high temperature region occurs with the
advent of double
fl
flames, which increases the rate of thermal NO
x
production. This
increase in thermal NO
x
might be possibly countered by the decrease in prompt
NO
x
production, and thus almost no net change in NO
x
concentration is evident.
EICO remains constant for
U
p
[
10, but for
U
p
10, CO emissions decreased
monotonically as expected. This supported more complete combustion, even
though the
fl
U
p for minimum NO
x
does not give the minimum CO. The HC
emissions also showed a trend similar to EICO, but the
U
p for minimum NO
x
showed minimum HC emissions as well.
Semerjian et al. (
1979
) studied emissions of NO
x
, CO and Total Hydrocarbons
(THC) with respect to the local equivalence ratio for a
'
partially
'
mixed
fl
ame. For
U
¼
4, the THC and CO are maximum at the centre of the burner and decrease
towards the edges. This indicates that the fuel is rapidly oxidized at the burner
peripheries, since the temperatures are higher in these richer zones, while leaner
mixtures and lower temperatures at the centre of the burner favour an increase in
CO and THC production. The NO
x
concentration shows a reverse trend, being
minimum at the centre and higher at the peripheries, as due to lower temperatures,
lesser thermal NO
x
0
:
78, no
unburned hydrocarbons are detected, and CO is minimum at the centre, indicating
complete
is formed at the centre and vice versa. For
U
¼
0
:
combustion and leanest
fuel
air mixture. For
fl
ames between
-
0
0, the NO
x
levels are below the corresponding premixed levels while
the CO levels are correspondingly higher. The low NO
x
:
6
U
1
:
levels might be due to
convective and radiative heat losses near the walls.
Kim et al. (
2009
) studied the mixing effects of fuel and air in a lean premixing
mode (partial premixing but with a lean premixed mixture) and compared the
emissions of NO
x
and CO in each case. At higher equivalence ratios, since the
oxygen supply is reduced, the temperature of the reaction zone increases, which in
turn decreases the CO emissions. Also, the temperature rise favours thermal NO
x
formation. Both NO
x
and CO emissions are observed to be much lower with a lean
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