(44c) The Influence of Particle Scale Properties on Biomass Bulk Scale Properties

Biomass feedstock materials consist of complex particles that can often cause significant flow problems.  Substantial testing of energy yields or fuel conversion yields is done to validate a biomass conversion process.  While this is important to the feasibility of the energy process, factors contributing to the successful biomass energy process are strongly linked to the ability to handle the biomass in the required unit operations under various moisture contents and particle sizes.   There are several issues to consider.  Biomass feedstock is light and very compressible.  This causes issues when designing transport equipment because the size of that equipment must be large to deliver the required mass flow rate.  However, biomass materials also have moderate cohesive strengths and, when this property is combined with the light density, significant hang-ups can occur.  The capacity for a material to arch across an outlet is directly proportional to the unconfined yield strength – increasing as the bulk strength increases.  However, the arching capacity is inversely proportional to the bulk density – increasing as the bulk density decreases.  The same holds true for the ability of the material to from stable ratholes.  Typically, more finely cut materials have greater strength.  However, in biomass systems, the finer materials also have heavier densities.  Additionally, adding moisture to the biomass feedstock increases material strength, but also increases the bulk density.  Thus, the effect(s) that changing the particle size and/or shape or adding liquid may have on the material’s capacity to form arches and ratholes in process equipment is unclear.

This paper presents a systematic study of the likely effects and presents measurement data for cohesive potential of biomass feedstock as a function of size, shape, and moisture content.  It shows regimes that cause flow problems and those that do not produce flow issues.  Elastic properties of biomass particles were measured, along with the particle aspect ratio, and are correlated the strength information.   It was also discovered that materials that possess a greater bending moment also show increased strength.