Interaction of Inherent Minerals with Carbon during Biomass Pyrolysis Weakens Biochar Carbon Sequestration Potential

Biomass carbon could be sequestrated in the form of biochar, an aromatized carbon structure produced by pyrolysis. Inherent minerals are reactive constituents that interact with organic contents during pyrolysis, significantly affecting the properties of the pyrolysis product. Despite their importance, their influence on biochar–carbon sequestration has been rarely studied. For this study four types of biomass were selected: barley grass, peanut hull, cow manure, and sewage sludge to investigate the influence of inherent minerals on carbon conversion during pyrolysis. Results showed removal of inherent minerals shifts the peak biomass conversion to a higher temperature (370 °C) compared to that of biomass with inherent minerals being present (330 °C). It also led to reduced emissions of low-molecular-weight organic compounds. Compared to pristine biomass, more carbon (3.5–30.1%) could be retained in biochar along pyrolysis after removing inherent minerals. And it showed increased resistance to chemical and thermal oxidation decomposition, indicating higher carbon stability and therefore carbon sequestration potential. Instrumental analysis showed removal of inherent minerals facilitated disappearance of oxygen-containing functional groups such as C═O, O═C–O, and C–O, while promoting C–C/C═C bonds, indicating higher aromatization of biochars. This study suggested that removing minerals prior to pyrolysis can be a promising approach for strengthening carbon-sequestration potential of biochar.