Production and characterization of waste nutshells derived biocarbon through slow pyrolysis: an investigation on the effects of pyrolysis temperature

The nutshell waste streams (pine nut shells (PNS), walnut shells (WNS), and almond shells (ANS)) were effectively converted into biocarbon materials employing slow pyrolysis in a fixed bed horizontal reactor at 500 and 900 °C with a 10 °C min⁻¹ heating rate and 60 min holding time. The acquired biocarbon materials were then characterized to understand the influence of pyrolysis temperature on their ash content, elemental composition, thermal stability, graphitic content, functionality, surface morphology, and electrical conductivity. The obtained results established that the pyrolysis temperature has a leading impact on the yield and physicochemical properties of biocarbon. Increasing the pyrolysis temperature of PNS, WNS, and ANS from 500 to 900 °C, respectively, decreases the biocarbon yields from 32, 28, and 28 wt.% to 28, 24, and 25 wt.%. Furthermore, the increasing pyrolysis temperature increases the carbon content and purity of PNS, WNS, and ANS biocarbon due to the elimination of oxygen. Thus, the electrical conductivity of PNS, WNS, and ANS biocarbon acquired at 900 °C was significantly higher than 500 °C. Overall, it can be surmised that biocarbon prepared at a higher temperature is preferable for material applications due to its greater characteristics.