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

Perennial global energy demands and the need to cut on emissions from fossil fuels has paved the way for alternative energy sources such as raw biomass and animal waste. In addition, there is limited research in deducing the impact of blending waste material to improve its characteristic properties using pressurized torrefaction. In this regard, this research explored the benefits of blending animal waste with wood chips to produce a bio-coal with improved 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 a positive adjustment in temperature and pressure improved the fixed carbon content of the blend almost threefold from 19.87–66.90% and the higher heating value (HHV) to 27.32MJ/kg from 13.90MJ/kg at mild torrefaction temperature of 280 ^o C and gas pressure of 4MPa. Wood chips had an HHV of 27.00MJ/kg upon treatment 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 substances and moisture. Moreover, pressurized torrefaction facilitated the decomposition of cellulose at a lower temperature than the typical range of 315-400 ^o C if conducted at atmospheric pressure. Furthermore, the study showed that to overcome the drawbacks of having a lower fixed carbon and HHV for animal waste, blending animal waste with wood chips was effective in synthesizing bio-coal with improved properties like some coals.