Particle shape and Stefan flow effects on the burning rate of torrefied biomass

Abstract

Experiments and subsequent analysis were conducted to gain insight into combustion phenomena of raw and torrefied biomass chars, and to describe the char combustion phase with respect to their frequently discussed non-spherical particle shape. Torrefied beech wood and miscanthus were burned in a laminar flow reactor at high heating rates (104–105K/s). A stereoscopic imaging pyrometry system was used to measure particle temperatures, sizes and shapes of sub-mm particles in-flight. Additionally, partially reacted samples were collected and analyzed by means of scanning electron and digital microscopy. A good agreement was found between collected and in situ measured particle sizes and aspect ratios. The initially high aspect ratio of the aspherical biomass particles was observed to decrease during the burnout progress. Reaction rates were derived by conducting an energy balance around a burning particle, accounting for the effects of both the particle shape and Stefan flow. The char burnout rate was found to decrease with torrefaction intensity of beech wood; beech wood showed a slightly higher reaction rate than torrefied miscanthus being treated with the same thermal parameters. The effect of Stefan flow was found to be of minor relevance during combustion in air; neglecting it, in the case of small particles and/or particles with high aspect ratios, particularly those burning in elevated oxygen atmospheres, leads to underprediction of particle temperatures and hence, burnout times.