Effects of particle shape and size on devolatilization of biomass particle

Experimental and theoretical investigations indicate particle shape and size influence biomass particle dynamics, including drying, heating rate, and reaction rate. Experimental samples include disc/flake-like, cylindrical/cylinder-like, and equant ( nearly spherical) shapes of wood particles with similar particle masses and volumes but different surface areas. Small samples ( 320 mu m) passed through a laboratory entrained-flow reactor in a nitrogen atmosphere and a maximum reactor wall temperature of 1600 K. Large samples were suspended in the center of a single-particle reactor. Experimental data indicate that equant particles react more slowly than the other shapes, with the difference becoming more significant as particle mass or aspect ratio increases and reaching a factor of two or more for particles with sizes over 10 mm. A one-dimensional, time-dependent particle model simulates the rapid pyrolysis process of particles with different shapes. The model characterizes particles in three basic shapes ( sphere, cylinder, and flat plate). With the particle geometric information ( particle aspect ratio, volume, and surface area) included, this model simulates the devolatilization process of biomass particles of any shape. Model simulations of the three shapes show satisfactory agreement with the experimental data. Model predictions show that both particle shape and size affect the product yield distribution. Near-spherical particles exhibit lower volatile and higher tar yields relative to aspherical particles with the same mass under similar conditions. Volatile yields decrease with increasing particle size for particles of all shapes. Assuming spherical or isothermal conditions for biomass particles leads to large errors at most biomass particle sizes of practical interest. (C) 2008 Elsevier Ltd. All rights reserved.