Synthesis and Fluidization of Wood Powders Application to biofuel production
Résumé
Synthesizing fuels from biomass sounds a promising path to reduce the overall green house gases emissions and replace the oil-based fuels. Among the main conversion routes, i.e. biological and thermochemical, the latter presents the advantage of being in the continuation of traditional process engineering, thus more inclined to be rapidly industrialized. Here, we investigate two key points of the process consisting in the transformation of solid wood into suitably sized particles, their transport and injection into an entrained flow gasifier and the Fischer-Tropsch conversion of the resulting syngas in liquid fuel. Because of the very large elasticity of ligno-cellulosic materials, the size reduction by grinding requires too much energy to be economically competitive. In order to increase the wood fragility, and thus decrease the energy consumption, the biomass is preliminarily torrefied (i.e. heated up to 300°C under inert atmosphere). In this section, we present new results correlating the required specific energy, the size of the grid holes used in the knife-mill, the torrefaction intensity and the resulting particle size. For two types of wood (beech for hard wood and spruce for softwood), it is evidenced that the grinding energy decreases exponentially when the particle size increases. Both the prefactor and the characteristic length linearly decrease when the torrefaction becomes more severe. The anisotropic microstructure of ligno-cellulosic materials is the cause of the elongated particles morphology. The poor flowability associated with this particle shape makes the pneumatic transport and injection problematic, which is clearly an obstacle to the industrialization of the process. Indeed, to avoid jamming, larger flow rates of inert gas must be used to convey the particles, with an increasing cost of gas separation at later stages. Here, we present new experimental results showing how flowability is modified by the particles size and shape, which in turn depend on the wood species and the torrefaction intensity. Due to its complex nature, the flowability is characterized in three configurations. The jammed-unjammed threshold is evaluated by shear tests performed on a packed bed. The dense quasistatic flowability is evaluated using a powder rheometer that measures the energy required by a rotating blade to circulate through the bed. The fluidization ability is evaluated at two different scales: in the powder rheometer in presence of an upwards air flow and in a standard fluidization column. Results show that the flowability of torrefied wood particles is very poor and very dependent on the wood species (hardwood or softwood). Due to particles interlocking, the bed is highly porous. This is evidenced by the pressure drop which is about 4 times smaller than expected by the Wu & Yen correlation for compact particles of same size. Also, this interlocking is the cause of an apparent cohesivity, which manifests itself by a channeling tendency and a minimum fluidization velocity about 5 to 10 times larger than expected by the Wu & Yen correlation. The significant difference between spruce and beech flowability is attributed to the particles morphology.