Sustainable management of anaerobic digestate through torrefaction into bioenergy: Mass and energy balance, performance indices, thermal-pyrolysis behavior

This study examines the torrefaction of raw digestate (RD) to upgrade properties of product followed by thermal-pyrolysis behaviour analysis of RD and torrefied product. Results showed that key torrefaction indices mainly mass yield (MY%), energy yield (EY%), higher heating ealue (HHV), energy-mass co-benefit index (EMCI) were affected by torrefaction reaction temperature. As temperature increased 200 °C to 300 °C, MY% was decreased from∼95 % to ∼69 % while EY% initially enhanced till 260 °C, then became constant till 300 °C. Torrefaction enhanced fixed carbon and HHV by 14.3–86.7 % and 15.8–35.8 % as compared to RD as temperature increased from 200 to 300 °C, however, ash content increased. Torrefaction of RD at 240 °C for 30 mins provided best results in terms of MY%, EY% and HHV with tradeoff between fixed-carbon and ash-content. Furthermore, thermal-pyrolysis behaviour of RD and TD240 was investigated by model-free and model-based kinetic models to examine the kinetics primary reactions under thermal decomposition. Estimated activation energies (Ea) reduced from 129.2-225.9 kJ/mol, 122.4–203.5 kJ/mol, and 127.3–227.4 kJ/mol for RD to 107.5–132.4 kJ/mol, 110.4–132.8 kJ/mol and 106.2–132.7 kJ/mol for TD240 using Friedman, Kissinger-Akahira-Sunose (KAS) and Ozawa-Flynn-Wall (OFW) models respectively. Activation enthalpy (ΔH) obtained 123.9–190 kJ/mol and 108.4–127.6 kJ/mol for RD and TD240. Furthermore, mass and energy balance showed the process has 97.65 % self-efficient without any external energy. Overall, effectiveness of torrefaction process showed to upgrade the existing biogas plants.