Effect of high-frequency plasma pyrolysis on the physicochemical properties of biochar

Abstract

Biochar has distinct physicochemical properties, making it a subject of considerable significance in modern environmental research. The choice of production route is a key factor that affects the properties of biochar and its potential applications. This study explored the effect of plasma pyrolysis on the physicochemical properties of biochar. The resulting biochar product was then subjected to a comprehensive characterization. Briefly, proximate composition and thermal behavior were characterized using thermogravimetric analysis (TGA). Ultimate analyses were conducted via an elemental analyzer. The morphological changes were characterized by scanning electron microscopy (SEM), while specific surface area and pore size distribution were quantified using the Brunauer-Emmett-Teller (BET) model. Three different and representative biomasses, viz., Miscanthus, brewery spent grain (BSG), and smoke wood, were used as feedstock. As a reference, conventionally pyrolyzed biochar of each biomass type was utilized. The results show that plasma pyrolysis significantly reduces the volatile content of biochar across all biomass types by up to 68% while increasing ash content by as much as 9%, with smoke wood having the maximum volatile reduction and miscanthus having the highest change in ash content. TGA analysis revealed that plasma pyrolyzed biochar samples exhibited higher thermal stability, evidencing the late on-set temperature and high ash content, aligning with the proximate results. Interestingly, the specific surface area of plasma pyrolyzed biochar was found to be significantly lower, ranging from 7.07 to 9.06 m2/g, than the conventional counterpart, ranging from 287.15 to 488.39 m2/g. The total pore volume follows the same trend. However, a significantly broad pore size distribution was observed for plasma pyrolyzed biochar, attributed to the deformation of a network of microstructures that led to a low surface area and pore volume but enlarged pore size. The surface morphology further supports this notion since it reveals the lack of microporous structure and the predominance of meso- and macropores. A consistent increment of carbon content and aromaticity, with increases ranging from approximately 4% to 18% and a reduction of oxygen by up to 15% across all plasma pyrolyzed biochar, was observed. The findings of this study demonstrate that high-frequency plasma pyrolysis significantly affects the physicochemical properties of biochar, with slight differences across biomass types.