Identification of Suitable Biomass Torrefaction Operation Envelops for Auto-Thermal Operation

Auto-thermal operation of biomass torrefaction can help avoid additional heat investment and the associated costs to the system. This paper defines the boundaries of practical auto-thermal operation for biomass torrefaction, which provides a possible range of operating conditions to guide the design of the commercial process. This work provides a general method for relating the feedstock-specific parameters to the energy balance and pre-diagnosing the potential of auto-thermal for different biomass torrefaction and pyrolysis systems. This work considers changes of bBoth solid and gas thermal properties during the torrefaction process and energy balance under various torrefaction conditions and their influences to the torrefaction system energy balances are considered. Key parameters that influence the process auto-thermal operation are analyzed, which include torrefaction reaction heat, torrefaction conditions, drying method, biomass species, and inert N2 flowrate. Equations for of torgas and biomass high heating values, as well as the torrefaction reaction heat at different operating conditions are developed. It is found that torgas and biomass HHVs increase with torrefaction temperature and biomass weight loss. Torrefaction reaction heat has a linear relationship with the biomass weight loss, with a positive slope at 250oC to 260oC, and a negative slope at 270oC to 300oC, which indicates that the torrefaction shifts from endothermic to exothermic at a torrefaction temperature of ~270oC. Applying of advanced drying technology and avoiding use of N2 can help the system achieve auto-thermal operation at lower torrefaction temperatures and residence times, thus leading to a higher process energy efficiency and product yield. The auto-thermal operation of the torrefaction system varies between different biomass species and different operating conditions. Present work provides a general method for relating the feedstock-specific parameters to the energy balance and pre-diagnose the potential of auto-thermal for different biomass torrefaction and pyrolysis systems. This is the first work to relate the micro level element changes of biomass to the macro process level energy balances of the torrefaction system. This work is important in design and operation of the torrefaction system in both pilot and industrial scales to improve process efficiency and predict product quality in a reliable and economic manner.