Pressurized torrefaction of waste biomass to improve bio coal quality: Synergistic effect between animal waste and wood chips

This study aims to investigate the effect blending waste material to improve its fuel properties using pressurized torrefaction. This research explored the benefits of blending animal waste with wood chips to produce a bio-coal with improved fuel properties. The process conditions investigated were temperature and pressure intervals of 200°C to 280°C and atmospheric pressure (AP) to 4MPa, respectively. The results showed that an increase in temperature and pressure improved the fixed carbon content of the blend almost threefold from 19.87 % to 66.93 % and the higher heating value (HHV) to 27.32MJ/kg from 13.90MJ/kg at mild torrefaction temperature of 280°C and gas pressure of 4MPa compared to atmospheric pressure conditions and the lowest temperature investigated. The HHV increased primarily due to a release of bound and unbound moisture and volatile matter. Wood chips had an HHV of 27.00MJ/kg at a torrefaction temperature of 280°C due to the decomposition of hemicellulose and cellulose which enhanced the thermal stability, fixed carbon content and calorific value. However, animal waste had the least incremental increase in HHV (16.45MJ/kg) due to a high initial content of volatile matter and moisture. The improved properties of the blend of materials indicated that pressurized torrefaction was effective in increasing fixed carbon content through secondary polymerization reactions. Moreover, it facilitated the decomposition of cellulose at a lower temperature than the typical range of 315-400°C if conducted at atmospheric pressure. This study elucidates the notable role of the synergistic effects of blending feed materials prior to torrefaction towards improving the properties and pyrolysis performance of biomass components.