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

Chapter 7
composition on the condition of biomass used, the type of gasifier and conditions of
pressure and temperature.
tel-00623090, version 1 - 13 Sep 2011
Another issue evolved in this work is the combustion, which detailed bibliographic
revision allows to verify that in premixed flames, the laminar burning velocity and flame
structure data can be extremely useful in the analysis of fundamental processes such
as ignition, NO, and soot formation, and flame quenching. Also, turbulent flame models
often prescribe the turbulent burning velocity as a function of laminar burning velocity.
Thus, detailed information describing the dependence of the laminar burning velocity,
flame thickness, ignition temperature, heat release rate and flame quenching on
various system parameters can be a valuable diagnostic and design aid. Burning
velocity is a physicochemical constant for a given mixture. It is the velocity, relative of
unburned gas, which a plane, one-dimensional flame front travels along the normal to
its surface. Clearly, it is the volume of combustible mixture, at its own temperature and
pressure, consumed in unit time by unit area of flame front. It is independent of flame
geometry, burner size and flow rate. The experimental methods for burning velocity
determination are described with emphasis for the constant volume and constant
pressure methods. In the constant pressure method the laminar burning velocity and
Markstein length are deduced from schlieren photographs. Moreover, any experimental
or computed value of laminar burning velocity should be associated with a value of the
flame stretch rate. Ideally, the stretch-free value of the burning velocity should be
quoted and the influence of stretch rate upon this value should be indicated by the
value of the appropriated Markstein length. This is the main reason of the increasing
use of the constant pressure method in which the stretch rate is clearly defined. The
main advantage of the constant volume method for determining the burning velocity is
the possibility of exploring a wide range of pressures and temperatures with one
explosion. This is the main reason of its utilization for burning velocity determination in
engine conditions.
Following the biographic revision, the combustion characterization of typical syngas-air
mixtures was initiated by the determination of the flammability limits in spherical
chamber. These results show that the pressure has a definite effect on flammability
limits of the syngas-air mixtures reducing the flammable region in the lean side. The
syngas combustion characterization continues with the laminar flame characteristics to
four equivalence ratios (0.6, 0.8, 1.0 and 1.2) within the flammability limits. The
influence of stretch rate on flame was determined by the correspondent Markstein and
Karlovitz numbers. Combustion demonstrates a linear relationship between flame
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