Mineral Coating Enhances the Carbon Sequestration Capacity of Biochar Derived from Paulownia Biowaste

Abstract

Biochar holds great promise for carbon sequestration but is restricted by high costs. Here, we introduced the water–fire coupled method and developed a mineral coating technique for biochar production from paulownia waste (Paulownia fortune). Exposure time and mineral (lime) coating were assessed for their impacts on biochar properties. The former had a dominant adverse effect on carbon content, specific surface area, and carbon capture capacity of the biochar. In contrast, the latter alleviated the adverse impact on carbon capture capacity and specific surface area, the highest being 67.07% and 176.0 m2 g−1, respectively. Without a mineral coating (B), biochar functional groups reduced at the exposure time of 0–4 min (-COOH from 0.50 to 0.19 mol/kg, phenolic-OH from 0.43 to 0.14 mol/kg). In contrast, a mineral coating (B-Ca) increased -COOH from 0.25 to 0.83 mol/kg and phenolic-OH from 0.19 to 0.72 mol/kg. The pyrolysis process with a mineral coating is conceptualized as (1) wrapping the paulownia branch with the mineral, (2) enabling oxygen-limited pyrolysis inside the branch, and (3) ending the pyrolysis with water to form biochar. Ca2+ played multiple functions of ion bridging, complexation, and reduction of COx gas formation, thus enhancing the carbon capture capacity (the ratio of C in biomass converted to biochar) to 67%. This research would improve the feasibility of biochar use for carbon sequestration and climate change mitigation.