Environmental Engineering Reference
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Fig. 6 Computed adiabatic
flame temperatures of
methane-air and two biogas-
air mixtures. Pressure = 1 atm
and initial
temperature = 500 K
constituents are CH
4
and CO
2
with small traces of H
2
O and N
2
. This section provides
a brief overview of the fundamental combustion and emission properties of biogas.
Biogas has lower energy content compared to natural gas. For example, the
volumetric heating values of natural gas (94 %CH
4
) and biogas (60 %CH
4
/40 %
CO
2
) are 38.6 and 25 MJ/m
3
, respectively. This has consequences for using biogas
in natural gas-
red combustion devices, since lower heating value implies higher
feeding rates and lower
fl
flame temperatures. Figure
6
compares the computed
adiabatic
flame temperatures for methane-air and two representative biogas-air
mixtures. The compositions of these two mixtures, which represent the common
feedstock, are 68 %CH
4
/26 %CO
2/
/5 %H
2
O/1 %N
2
and 60 %CH
4
/33 %CO
2/
/6 %
H
2
O/1 %N
2
, respectively. As indicated, the biogas
fl
fl
flame temperature is about 100
-
200 K lower than that of methane, depending upon the feedstock biogas. Lower
temperatures imply lower
fl
flame speeds and thermal NO for biogas
fl
ames compared
to those for methane
flames. The comparison of laminar burning speeds for freely
propagating methane and biogas
fl
fl
flames is shown in Fig.
7
, which shows the
fl
ame
speed as a function of equivalence ratio and pressure. The
ames were computed
using the CHEMKIN software and the GRI-3.0 kinetic mechanism. As expected,
fl
fl
flame speeds are lower for biogas-air mixtures compared to those for methane-air
mixtures. The effect of pressure on
fl
flame speed is qualitatively similar for all the
three cases shown, with the
first decreasing sharply and then increasing
relatively slowly with increase in pressure.
Since biogas is potentially a cleaner and sustainable alternative to natural gas, it
is relevant to analyze the emission characteristics of methane and biogas
fl
ame speed
flames in
different combustion regimes. Figure
8
from Quesito et al. (
2013
) compares the
computed NO, NO
2
, and C
2
H
2
pro
fl
les in methane and biogas-air PPFs. These
fl
flames were simulated in a counter
fl
ow geometry at fuel stream equivalence ratio
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