Biocarbon graphenization processes and energy assessment. A review

Reducing energy consumption and environmental impact in the graphenic material production is of foremost importance. While using lignocellulosic biomass is gaining momentum, processing such a non-graphitizable resource is energy consuming as it requires a thermal treatment above 2000 °C. Three alternative carbonization routes are analyzed: catalytic, hydrothermal pre-treatment and solar. We discuss the sustainability of the biocarbon production as compared to conventional high temperature biomass carbonization. The mechanisms of carbonization and graphenization is unveiled using a variety of characterization methods from macroscopic to nanoscopic scale. In catalytic graphenization, it is showed that metal carbides play a crucial role as intermediate phases in the graphenization mechanism. Solar-driven pyrolysis focused intense radiative flux resulting in an improve of the growth and flattening of graphene layers in biocarbon. For each process considered, the operating conditions can finely be tuned to control the graphenization degree, and successfully convert non-graphitizable lignocellulosic biomass into graphenic biocarbon. This review provides new insights on mechanisms in relation to the energy and environmental assessment. For calcium-doped biomass we estimate an energy saving of 22 % when working at 1600 °C compared to carbonization of raw biomass at 2000 °C attributed to the formation of calcium carbide around 1300 °C. Hydrothermal pre-treatment provides a carbon pre-structuration which could save up to 85 % of pyrolysis energy input by lowering the graphenization temperature to 1200 °C, while solar pyrolysis consumes half the energy as compared to the conventional one. This drastically reduces the environmental impact of the biocarbon production.