Etude de la combustion de gaz de synthèse issus d'un processus de ...

Chapter 4
The values of laminar burning velocity reported by Natarajan et al. (2007) for syngas
(40H 2 -40CO-20CO 2 ) (dot symbols in Fig. 4.24) show that CO 2 has higher influence on
the reduction of the burning velocity than N 2, when compared with the (40H 2 -40CO-
20N 2 ) mixture. The reason for the substantial reduction on burning velocity when
dilution is made with CO 2 (37.28 J/mol K) instead of N 2 (29.07 J/mol K) is the increase
in heat capacity of the mixture. Consequently, the flame temperature also decreases.
Adiabatic flame temperature obtained by Gaseq gives 2535 K for the mixture
comprising CO 2 and 2624 K for the mixture comprising N 2 .
The values of laminar burning velocity reported by Huang et al. (2004) for syngas
(28H 2 -25CO-47N 2 ) (circular symbols in Fig. 4.24) can be seen to be higher than those
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obtained for the syngas in the current study; this is associated with the lower H 2
content, greater N 2 content and the presence of CO 2 in the typical syngas compositions
considered in this work. Similar behaviour of the laminar burning velocity is found
between (30H 2 -30CO-40N 2 ) and (28H 2 -25CO-47N 2 ) mixtures given its analogous
composition.
Downdraft syngas has a similar composition as 20H 2 -20CO-60N 2 mixture. Therefore,
the comparison shows that for very lean mixtures (φ=0.6) the burning velocity values
are similar. However, an increasing difference in burning velocity is observed for latter
equivalence ratios. Thus, emphasis the influence of the H 2 amount in the mixture,
which in this case is only 3% by volume lower and the increased heat capacity of the
mixture due to the dilution by CO 2 (13%) instead of N 2 .
It can be observed that the magnitude of laminar burning velocity for the typical syngas
compositions is similar to that of a mixture comprising 5%H 2 /95%CO, although the
value of
0
Su
of the former peaks at a lower equivalence ratio than that of the latter. The
heat value of this mixture is more than three times higher than the typical syngas
composition. In opposite, the air-fuel ratio is about the double. Thus, the energy content
per unit quantity of mixture (air + fuel) introduced in the chamber is only marginally
lower when using typical syngas compositions.
4.1.2 Constant volume method
For a spherical flame, laminar burning velocity is a function of radius because of its
dependency on flame curvature (Markstein, 1964). The stretched laminar burning
velocity, S u , at a given radius can be calculated by the pressure history of combustion
according to Lewis and von Elbe, (1987), as follows:
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