Two-step synthesis of biochar using torrefaction and microwave-assisted pyrolysis: Understanding the effects of torrefaction temperature and catalyst loading.

The study focused on synthesizing the biochar from sawdust using torrefaction followed by pyrolysis. Sawdust was torrefied at different temperatures (125 °C, 150 °C, and 175 °C) using a conventional hot air oven. The obtained torrefied biochar was subjected to Microwave-assisted pyrolysis at a power of 300 W for 10 min. Graphite was used as a susceptor, and KOH was used as a catalyst. The maximum biochar product yield varied from 24 to 48 wt% and increased with torrefaction temperature. The average heating rates ranged from 54.5 to 74.6 °C/min. At 10 g of KOH, higher heating rates were obtained. The pyrolysis index analyzed varied between 97.5 and 111.5 and decreased with the increase in torrefaction temperature. The obtained biochar was analyzed using SEM, BET, XRD, FTIR, ICP-OES, and Raman spectroscopy. Porous structure formation enhanced, and the concentrations of Ca, Al, and Fe decreased with the increase in torrefaction temperature. [Display omitted] • Torrefaction pretreatment played an important role on biochar yield and composition. • Biochar was produced using microwave-assisted pyrolysis of pretreated sawdust. • Increase in the torrefaction pretreatment temperature enhanced the biochar yield. • Biochar was characterized by ICP-OES, XRD, BET, SEM, FTIR, and Raman spectra. • The pyrolysis index was for the microwave-assisted pyrolysis was calculated. [ABSTRACT FROM AUTHOR]