Improvement of Pelletability of Woody Biomass by Torrefaction under Pressurized Steam

The combination of torrefaction and pelletization offers the potential to produce fuels with high energy densities on both mass and volume bases as well as excellent storage and transportation characteristics from biomass. Conventional dry torrefaction, performed in inert atmosphere, inevitably decreases the pelletability of biomass because of the occurrence of a dehydrative reaction and resulting formation of char that inhibits densification in the pelletization. The present study revealed beneficial effects of pressurized steam on the production of torrefied woody biomass with high pelletability. Torrefaction of hardwoods was carried out at 180-250 degrees C in the presence of saturated steam (pressurized steam torrefaction, PST), and the tensile strength of pellets prepared from torrefied biomass was investigated. PST considerably improved the pelletability of biomass, producing pellets with tensile strength higher by 5.2 times than those from the original biomass. Such improvement in the pelletability was comparable to that by wet torrefaction (torrefaction in compressed water). Meanwhile, PST showed higher energy densification factors on both mass and volume bases than wet torrefaction at mass yields around 70%. The biomass after PST contained a 10-28 wt % acetone-soluble component, which contributed to a limited part of the improved pelletability. The improvement mainly arose from the acetone-insoluble component, which had undergone hydrolytic degradation under pressurized steam rather than charring.