micro-Raman indicates biochar has similar stability and structural features as natural fusinite and semifusinite

Abstract

Semifusinite and fusinite macerals are formed through carbonization of plant material in naturally occurring wildfires. Both macerals, belonging to the inertinite maceral group, are carbon-rich and oxygen-poor, and are typical constituents of charcoal. The charring of plant material into charcoal is a naturally occurring process that effectively stores carbon over geological timescales. The stability of carbon in biochar formed by pyrolysis of biomass are determined by its degree of carbonization, and long-term storage in soil assumes that the biomass has been completely transformed into inertinite biochar. Inertinite biochar is thought to have a structural composition comparable to highly stable natural fusinite, whereas less carbonized biochar is similar to less stable semifusinite. In this study, micro-Raman spectroscopy was employed to examine the structural composition of 16 biochar samples produced at different temperatures from various lignocellulosic feedstocks, as well as one natural semifusinite isolated from a coal deposit. The estimated carbonization temperatures, representing the actual internal heating temperatures experienced by the biomass, ranged from 326 to 825 °C. The micro-Raman results of the biochar samples were compared with previously published micro-Raman data on naturally formed semifusinite and fusinite. The findings show that biochar, semifusinite, and fusinite likely undergo similar structural evolution with increasing temperatures, suggesting that the process of pyrolyzing biomass into biochar may mimic the natural charring process that occurs during wildfires. Though additional work could help to validate benchmarks, these results support the idea that biochar stability is similar to that of fusinite and semifusinite and indicate its potential as a long-term storage solution in the context of the geological carbon cycle. Furthermore, the results indicate that in addition to the established inertinite benchmark (IBR_o2%; R_o = 2 %) for determining biochar stability, up to four new micro-Raman benchmarks can be used to define inertinite biochar: (1) D3-band position: 1460 cm^‐1 or higher, (2) D1/G amplitude ratio: 0.80 or higher, (3) Raman band separation (RBS): 250 cm^‐1 or higher, and possibly (4) G-FWHM (full width at half maximum): 72 cm^‐1 or lower. The availability of multiple benchmarks that can indicate biochar stability could provide tools for evaluating biochar long-term storage potential, thus reinforcing carbon dioxide removal (CDR) via biomass waste pyrolysis as a viable technique to mitigate climate change.