downdraft moving-bed gasifier has been used by the
authors to carry out biomass gasification (Fig. 1). Biomass is
introduced by the upper part and is piled inside the gasifier. Air
is introduced through orifices at a height that establishes the
oxidation zone. The fuel to air ratio in the gasifier is kept
always below the stoichiometric (air defect). However the
operating range of the fuel to air ratio modifies the composition
of the produced gas and then its properties such as the energy
content (heating value) and of other types (density, combustion
speed). The heating value of the stoichiometric mixture of
gasification gas and air that feeds the engine should be
maximised. The rest of the properties of the gasification gas
are also important, as the laminar combustion speed or the
tendency to auto-inflammation.
The fuel to air ratio F is the ratio between the amount of fuel
and the amount of air that gets into a particular device, usually
measured and expressed in terms of mass flow rates. The value
Fs for the stoichiometric reaction can be used as a reference: