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

Chapter 2
Table 2.8 shows a comparison performed by Javier et al. (1999) based on results from
Herguido et al. (1992), Javier et al. (1997) and Narváez et al. (1996) in order to
determine the effect of gasifying agent on other results like tar content in the produced
gas. Three basic ratios were used for comparison of results using different gasifying
agents in a bubbling fluidized bed gasifier running with small chips of pine:
- Equivalence ratio (φ) for air;
- Gasifying ratio (GR) [(H 2 O+O 2 )/Biomass (kg/h)/(kg dry-ash free/h)], for steam-
O 2 mixtures;
- Steam to biomass ratio (SB) [H 2 O/Biomass (kg/h)/(kg dry-ash free/h).
Table 2.8 – Effect of gasifying agent on syngas composition
Parameter Air (φ=0.3) Steam/O 2 GR = 0.9 Steam SB=0.90
tel-00623090, version 1 - 13 Sep 2011
H 2 (vol.% dry basis) 8-10 25-30 53-54
CO (vol.% dry basis) 16-18 43-47 21-22
LHV (MJ/m 3 , dry basis) 4.5-6.5 12.5-13.0 12.7-13.3
Ygas (m 3 , dry basis/kg daf) 1.7-2.0 1.0-1.1 1.3-1.4
Y tar (g/kg daf) 6-30 8-40 70
Tar (g/m 3 ) 2-20 4-30 30-80
Gasification with air produces a higher syngas yield. Tar yields are quite different
between the gasifying agents being the lowest values obtained with air.
2.4. Concluding remarks about biomass gasification
Gasification is a versatile thermochemical conversion process which produces a gas
mixture of H 2 , CO and CH 4 the proportions being determined by the use of air, oxygen
or steam as the gasification medium, with a concomitant range of heat values, low (4–6
MJ/Nm 3 ), medium (12–18 MJ/Nm 3 ) and high (40 MJ/Nm 3 ). Key parameters for
successful gasification are the feedstock properties (moisture, ash, alkalis and
volatiles) and feedstock pre-treatment (drying, particle size, fractionation and leaching).
Gasifiers are of two main types, fixed bed and fluidized bed, with variations within each
type and specific characteristics which determine the need for and extent of feedstock
preparation/pre-treatment.
For use in gas engines gas produced from a fixed bed, downdraft gasifier provides a
low tar gas, with a high particulates loading: as tar is a major contaminant for engine
operation and particulates can be relatively easily removed, this system is considered
best for fuelling gas engines. Extensive development of wood gas-fuelled IC engines,
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